Syrian troops held their fire in the hours after a U.N.-backed ceasefire took effect at dawn on Thursday, casting a silence over rebellious towns they had bombarded heavily in recent days.

But the lull did little to convince opposition activists and Western powers of President Bashar al-Assad's good faith in observing a peace plan agreed with international envoy Kofi Annan. In defiance of that deal, Syrian troops and tanks were still in position inside many towns, activists told Reuters.

It was a bloody night. There was heavy shelling on the city of Homs. But now it is calm, and there is no shooting, said Abu Rami, an activist in Syria's third city after the 6 a.m. (04:00 a.m. British time) deadline passed. Assaults on restive neighbourhoods had become more intense after Assad accepted Annan's timetable.

At the United Nations, Annan will brief the deeply divided Security Council at 03:00 p.m. British time. Western powers, though hesitant to intervene militarily, are lobbying Russia, a key ally for Assad, to drop its veto on other U.N. measures to pressure Syria into abandoning four decades of autocratic rule by the his family.

The 13-month crisis has pushed out pressure waves along a web of faultlines across the Middle East, pitting Sunni Arabs against Shi'ite Iran, and alarming Turkey, whose prime minister on Thursday cited his country's right to call on its NATO allies to defend a border where Syrian troops opened fire this week.

People contacted by telephone in the flashpoint provinces of Homs, Hama and Idlib, which have been under sustained shelling by Assad's forces over the past week, reported calm. An activist in Damascus said the capital was also quiet.

More than two hours have passed, and it's completely quiet across the country, said Rami Abdulrahman, director of the British-based Syrian Observatory for Human Rights (SOHR).

The Syrian government bars access to most independent media.

Like activists in other cities, the man who uses the cover name Abu Rami said there was no indication troops were withdrawing from Homs in accordance with the agreement.

There are no signs of a pull-back; the tanks, snipers and armed forces are still visible across the city, he said.

DIPLOMATIC BATTLE

British Prime Minister David Cameron, speaking after the lull was reported, said Assad was failing to abide by the terms endorsed by the U.N. Security Council last week: I feel an immense sense of frustration because the world has come together behind this Kofi Annan plan, he told the BBC.

This is a plan, remember, that is not just backed by those of us who have been pushing for action on Syria, it's also backed by China and Russia. And yet Assad is deliberately flouting it.

In an indication of how the Western leaders who intervened to help rebels topple Muammar Gaddafi in Libya last year are reluctant to do likewise in Syria, Cameron made it clear that the main thrust of Western efforts would be to persuade Moscow and Beijing to accept tighter punitive sanctions.

Now is the time to say to the Russians and Chinese, look at the man we are dealing with, look at the appalling way he is behaving, Cameron said. We need to go back to the U.N. and tighten the pressure, tighten the noose.

The opposition Syrian National Council is also lobbying for a strong international ultimatum to Assad if the peace fails.

Russia and China, alarmed by the way last year's Security Council resolution on Libya led to military intervention against a sovereign state, have vetoed attempts to penalise Assad, although the United States, European Union and Arab League have imposed their own economic and political sanctions.

China's Foreign Ministry called on the opposition to honour the truce, something the disparate rebel movements have said they are willing to do - although Western leaders and Annan's team have made clear the onus is firstly on the government.

China welcomes the government's relevant decision, which will help to ease tensions, the ministry said in a statement. China also calls on the Syrian armed opposition to immediately cease fire and implement Annan's six-point proposal.

TROOPS STILL IN CITIES

Annan has said he has Syrian assurances that the ceasefire would be respected, though his spokesman was at pains on Wednesday to stress that other elements of his plan must also be respected - notably Item 2, which calls for armed forces to withdraw from the vicinity of urban areas.

Syria's Defence Ministry said on Wednesday it would halt operations on Thursday morning but made no mention of an army pullback from cities and said it would confront any assault by armed groups. Attacks on opposition neighbourhoods over the last week have fuelled doubts it would comply with the truce.

The U.S. ambassador to the United Nations, Susan Rice, said the commitment could not be construed as complying with the former U.N. secretary-general's peace plan: The burden remains squarely on the Syrian regime and not the opposition in the first instance to meet its obligations in full and visibly under the Annan plan, Rice told reporters late on Wednesday.

The caveats in the letter are worrying and yet again cast into doubt the credibility of any such commitments. But nothing casts more doubt on the credibility of the commitments than the fact that commitments have been made and made and made and broken and broken and broken, she said, noting that attacks had intensified since Assad agreed to the plan two weeks ago.

Syria's U.N. ambassador, Bashar Jaafari, said his government was on board with the plan. But, he said on U.S. television, government forces would remain on alert to counterattack and that legally speaking, there is a big difference between declaring a ceasefire and putting an end to the violence.

The opposition was also sceptical of Assad's intentions.

The Defence Ministry announcement is a detour on Annan's plan, which clearly says he should pull back the tanks and end violence. We will wait until tomorrow and see. We will not act before tomorrow, Qassem Saad al-Deen, Free Syrian Army spokesman inside Syria, told Reuters on Wednesday.

A spokesman for Annan said an advance planning team that had been negotiating how U.N. observers would monitor the accord had left Damascus after a week. Its leader, Norwegian general Robert Mood, told Norway's NTB news agency he was cautiously optimistic but that there remained considerable difficulties.

Both sides are plagued by a very high degree of mutual suspicion, Mood said. It's terribly difficult for them to cross that abyss. A veteran of peacekeeping missions in the Middle East, Mood said he had mainly spoken to Syrian officials.

TURKEY CITES NATO

Turkey, hosting nearly 25,000 Syrian refugees, has been particularly alarmed, and Prime Minister Tayyip Erdogan raised the prospect of engaging its NATO allies to protect its borders after firing from Syria hit people in a refugee camp this week.

NATO has responsibilities to do with Turkey's borders, he was quoted as saying by Hurriyet newspaper on Thursday, citing Article 5 of the North Atlantic defence pact, which allows for a common response to an attack on the territory of a member state.

Ankara has urged the Security Council to adopt a resolution that would protect the Syrian people, saying Damascus had not kept its troop withdrawal pledge and had increased the violence.

Annan, at a news conference in Tehran on Wednesday, urged Assad's long-time ally Iran to help resolve the violence and warned of unimaginable consequences if it worsened further.

Iranian President Mahmoud Ahmadinejad, quoted by state news agency IRNA, urged an end to violence but accused the NATO powers of expansionist ambitions in the Middle East and said Tehran's Syrian ally should not be put under pressure.

NATO is not ashamed to say it wants to dominate the region and is trying to extend its domination eastward, IRNA said, quoting Ahmadinejad as insisting: The implementation of any plan in Syria should be free of pressures and interference, and all violence in that country should be stopped.

Iran has unstintingly backed Syria, the only Arab nation to support Iran in its 1980-88 war with Iraq and the conduit for Iranian arms to Lebanon's Shi'ite Muslim Hezbollah movement.

Syria, where Assad's Shi'ite-rooted Alawite minority dominates a Sunni Muslim majority, has become an arena for a sectarian-tinged regional contest between Shi'ite Iran and Sunni Arab rivals aligned with the West and led by Saudi Arabia.

Assad's forces have killed more than 9,000 people in the past year, according to a U.N. estimate. Damascus says rebels have killed more than 2,600 soldiers and security personnel.

(Additional reporting by Marcus George in Dubai, Louis Charbonneau and Michelle Nichols at the United Nations, Sui-Lee Wee and Sabrina Mao in Beijing, Paul Eckert in Annapolis, Stephanie Nebehay in Geneva, Oliver Holmes and Dominic Evans in Beirut Writing by Philippa Fletcher; Editing by Alison Williams and Will Waterman)