The Syrian army will stop operations on Thursday morning but remain on alert to confront any attacks, Syrian state television quoted a defence ministry source as saying.

'After our armed forces carried out successful missions in combatting the criminal acts of terrorist groups and imposed the authority of the state on its land, (it has been) decided to end operations starting tomorrow morning,' a TV newsflash said.

'Our armed forces remain on alert to confront any assault by armed groups against civilians and the security and armed forces,' it said.

The news item made no mention of the Thursday 0300 GMT deadline for a ceasefire set by the UN and Arab League peace envoy Kofi Annan and agreed to by President Bashar Al-Assad.

Earlier  troops pummelled opposition neighbourhoods in the city of Homs with heavy mortars, reports said.

'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 quarter. Sniper fire cracked out incessantly in the background. 

At least 12 people were killed on Wednesday, activists said.   

Western powers have scorned Assad's truce pledges to Annan, 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 UN 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. 

He said Annan had reported that Assad's troops were 'conducting roving military operations in population centres supported by artillery fire'. An estimated 1,000 people had been killed in the past week, Cameron said. 

US 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.  - Reuters