Syrian troops pummelled opposition neighbourhoods in the city of Homs with heavy mortars on Wednesday, hours after President Bashar al-Assad assured the United Nations he will respect a ceasefire with rebels due to take effect in less than 24 hours.

Activist videos posted on YouTube showed bombs crashing into the Khalidiya district, the latest target of Assad's drive to crush his opponents after troops overran the besieged rebel stronghold of Baba Amr in Homs a month ago.

Spouts of pulverised debris burst high into the air with each impact and plumes of dust and smoke drifted over the rooftops. The videos could not be independently verified and the Syrian government bars most independent media from the country.

The Syrian Observatory for Human Rights (SOHR), a British-based activist information organisation, said one person was killed in the morning attacks.

Mortar fire started at 7 this morning. I can hear one explosion every five minutes, said opposition activist Waleed al-Fares in Homs, where bombardment killed at least 26 people on Tuesday, the day Assad was supposed to have begun withdrawing troops, tanks and heavy weapons from urban centres.

Western powers have scorned Assad's truce pledges to U.N.-Arab League envoy Kofi Annan, but have come up with no effective policy to curb the bloodshed in Syria, given their own aversion to military intervention and the resistance of Russia and China to any U.N. Security Council action.

U.S. Secretary of State Hillary Clinton said she would meet Russian Foreign Minister Sergei Lavrov on Wednesday to seek a policy change from one of Assad's few foreign friends.

We will have another go at trying to persuade the Russians that the situation is deteriorating and the likelihood of regional conflict and civil war is increasing, she said.

China expressed deep worries about the violence in Syria and called for all sides to respect a ceasefire. Foreign Ministry spokesman Liu Weimin also described Annan's work as an important and rare opportunity to end the crisis.

Annan's plan is for now the only game in town and the former U.N. chief said it must be given a chance to work. His deadline, endorsed by the Security Council, is for a ceasefire by dawn.

If everyone respects it, I think by 6 in the morning (0300 GMT) on Thursday we shall see improved conditions on the ground, he said in Tehran, where he was asking Syria's staunchest regional ally to support his efforts.

BURNING HOUSES

But the Syrian military has so far stayed on the offensive, pursuing assaults on several anti-Assad strongholds, rather than withdrawing, as Annan's plan required them to do on Tuesday.

The SOHR said two people were killed in army raids in Deir al-Zor in the Euphrates river valley far to the east. Artillery shelled the Jebel Akrad area in the coastal province of Latakia.

Helicopters flew overhead and the army prevented the evacuation of people. There are reports of houses being destroyed by shelling while families are still in them, said Rami Abdelrahman, head of the SOHR.

In southern Deraa, birthplace of the 13-month-old revolt against four decades of Assad family rule, activists said many busloads of troops backed by armoured vehicles had flooded the city and were making house-to-house raids.

Activist Omar al-Hariri said he had never seen so many troops: The army is exploiting the ceasefire to arrest more dissidents than ever and security forces are burning houses.

Syrian government restrictions on independent media make it hard to assess accounts by officials and dissidents.

The SOHR said Syrian troops had killed 38 people on Tuesday, and rebels had killed 19 members of the security forces. The state news agency reported the funerals of 33 soldiers and security personnel the same day.

Anti-Assad rebels have said they will stop fighting if the Syrian military pulls back and ceases fire as promised.

Annan, at a news conference with Iranian Foreign Minister Ali Akbar Salehi, urged Iran to help resolve the violence and warned of unimaginable consequences if it worsened further.

I have received (Syrian) government assurances they will respect the ceasefire, he said, adding: I believe Iran can be part of the solution.

Salehi said Syrians should be able to have free elections contested by political parties, but reiterated Iran's opposition to any outside interference in Syria's affairs and made clear the Islamic Republic wanted Assad to stay in charge.

The opportunity must be given to the Syrian government to make changes, under the leadership of Bashar al-Assad, he said.

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.

Despite its public acceptance of Assad's narrative that he faces foreign-inspired terrorist subversion, Tehran has also reached out to his opponents, according to an adviser to Iran's parliamentary speaker quoted by local media on Tuesday.

It has recently made contacts with the Syrian opposition, which shows Iran's influential role in resolving issues, the Tehran Times quoted Hossein Sheikholeslam as saying.

For now, no end to Syria's agony is in sight.

The SOHR said on Tuesday more than 800 civilians had been killed since Annan announced on March 27 that he had obtained Assad's agreement to the peace plan.

More time means more blood, said the opposition Syrian National Council's spokeswoman Basma Kodmani. It is urgent to end the regime's repression and the regime itself.

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 at the United Nations, Sui-Lee Wee and Sabrina Mao in Beijing and Paul Eckert in Annapolis; Writing by Douglas Hamilton; Editing by Alistair Lyon and Alastair Macdonald)