Syria promised to observe a U.N.-backed ceasefire starting on Thursday, but its forces kept up fierce attacks on opposition neighbourhoods in the hours before the deadline.
A Syrian defence ministry source quoted on state television on Wednesday said the army would halt operations on Thursday morning, but would confront any assault by armed groups.
U.N.-Arab League envoy Kofi Annan said the Syrian government had also assured him it would stop fighting by the dawn deadline he has set for a cessation of hostilities.
It agreed to cease all military fighting throughout Syrian territory as of 6 a.m. (0300 GMT) tomorrow, Thursday, 12 April, 2012, while reserving the right to respond proportionately to any attacks carried out by armed terrorist groups against civilians, government forces or public and private property, Annan's spokesman Ahmad Fawzi said in a statement, quoting a letter from the Syrian Foreign Ministry.
Russia, a powerful defender of President Bashar al-Assad against Western and Arab pressure, said the rebels battling to oust him must honour the ceasefire too.
Insurgents, who lack a clearly coordinated command structure, have previously said they will stop shooting if Syrian forces pull back and observe the truce as promised. But few in the Syrian opposition believe Assad has any intention of complying with Annan's plan to end 13 months of bloodshed.
Annan, this is your ceasefire, ran the sarcastic voiceover on an activist video that showed a shopping mall engulfed in flames after it was hit in bombardment of the Juret al-Shayah district of Homs. Sniper fire cracked out in the background.
At least 12 people were killed on Wednesday, activists said.
Western powers, too, have scorned Assad's truce pledges, but so far lack an effective policy to curb the bloodshed, given their own aversion to military intervention and the resistance of Russia and China to any U.N. Security Council action.
Far from fulfilling their commitment, the regime has cynically exploited the window of diplomatic negotiations to crack down even harder on its own people, British Prime Minister David Cameron said during a visit to Indonesia.
Activist videos posted on YouTube showed bombs crashing into the Khalidiya district of Homs.
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 verified and the Syrian government bars most independent media from the country.
The Syrian Observatory for Human Rights (SOHR) said three people were killed in Homs. Shelling killed a man, woman and child in Qusair near the border with Lebanon. Three people were killed near Damascus, the British-based opposition group said.
Mortar fire started at 7 this morning. I can hear one explosion every five minutes, said activist Waleed al-Fares in Homs, where bombardment killed at least 26 people on Tuesday.
If Assad fails to respect a ceasefire, the world should unite against him, using an arms embargo and other sanctions, the main opposition group said, hours before the truce deadline.
The chances that by tomorrow the regime will implement or abide by the ceasefire are weak, as we all know, Syrian National Council spokeswoman Basma Kodmani said in Geneva.
We would like to see a unanimous decision by members of the Security Council that sends an ultimatum to the regime with a deadline that is not too far down the road that says on such and such a date enforcement measures will intervene, she said.
U.N. action would need the support of Russia and China, which have blocked previous Security Council draft resolutions on Syria, citing concerns about a Libya-style intervention that would breach Syrian sovereignty.
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.
Turkey, hosting nearly 25,000 Syrian refugees, said Foreign Minister Ahmet Davutoglu would discuss the Syria crisis with counterparts from the Group of Eight major nations on Wednesday evening, via a video conference call.
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 said his plan, endorsed by the Council, must be given a chance to work.
If everyone respects it, I think by 6 in the morning 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.
But the Syrian military has stayed on the offensive, pursuing assaults on several anti-Assad strongholds, instead of pulling back, as Annan's plan required them to do on Tuesday.
On Wednesday, an activist in the city of Hama said at least 20 armoured vehicles had moved into two central neighbourhoods, while an opposition supporter in Rastan, between Hama and Homs, said heavy shelling of the town began after the announcement by the Syrian government that it would respect the ceasefire.
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.
In Deraa, cradle of the revolt against four decades of Assad family rule, activists said 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.
PART OF THE SOLUTION
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.
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.
For now, no end to Syria's agony is in sight.
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.
This is a decisive moment, Cameron said, adding - in remarks clearly pointed at Moscow and Beijing - that the Security Council now had a clear responsibility to throw its weight behind Annan's plan and insist it is implemented.
(Additional reporting by Marcus George in Dubai, Louis Charbonneau 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 and Mohammed Abbas in Jakarta; Writing by Alistair Lyon; Editing by Alastair Macdonald)