Syrian tank and mortar fire hit the cities of Homs and Hama and insurgents killed six soldiers on Tuesday, opposition activists said, on the day President Bashar al-Assad was to withdraw his forces 48 hours ahead of a planned ceasefire.

The anti-Assad Syrian Observatory for Human Rights said the soldiers were killed in attacks on two checkpoints on the highway running through the remote eastern desert town of Marqada, south of the Turkish border.

There was no report of the attacks from the official Syrian news agency, SANA. Syria says it has already pulled back some of its troops from the cities in keeping with its undertaking to United Nations and Arab League peace envoy Kofi Annan.

His peace plan calls for rebel forces to stop shooting as the army withdraws so that all forms of violence cease at dawn on Thursday.

The Observatory, a British-based activist information centre that has collated reports on the violence in Syria for the past year, said there was no clear sign of troop withdrawals.

It reported a clash between rebels and army in Deraa in the far south and said the battered opposition stronghold of Khalidiya in the city of Homs was again heavily shelled.

Activists said the Homs bombardment began at breakfast time.

Shelling woke me this morning at 8.30 and I can now hear one shell every 10 minutes or so, said Waleed Fares, describing what he said was mortar rounds striking neighbourhoods in the centre and east of Homs, hub of the 13-month-old uprising.

One 90-second video recorded surreptitiously by an activist showed a large group of soldiers in combat gear at a normally busy intersection in Homs. There was no one else on the streets.

EXPLOSIONS AT NIGHT

In Hama, Manhal Abu Bakr heard shelling start up overnight and said tanks were still patrolling the city.

At 2 a.m. we heard two shells fall and the sound of tanks moving around the streets, he said.

There is no gunfire now. They shell us at night so that it is hard to film, he said via Skype. Internet video, which Abu Bakr said was filmed in Hama overnight, showed nighttime explosions in a built-up district.

There were no immediate reports of civilian casualties. The Observatory said most other cities were relatively calm on Tuesday after heavy bloodshed in recent days.

There were no immediate reports of action from the rebel Free Syrian Army, whose commanders have said they will order a ceasefire only if they are satisfied that Assad's forces have indeed pulled back and stopped offensives.

The Observatory said there was an overnight bombardment in the town of Mara in Syria's northern province of Aleppo.

In Douma, a suburb of the capital Damascus, an activist said tanks were still on the fringes of town on Tuesday morning.

Residents of Deraa, where the popular revolt against Assad erupted in March 2011, reported sporadic gunfire.

Security is everywhere and you feel they have redeployed in key locations, said Nayef Hassan, an engineer.

Security forces and the army remained stationed in Deraa, said an activist who called himself Abu Firas, and security checkpoints still separated districts of the old city.

The troops at checkpoints are appearing in strength to say 'we are present', he said.

(Reporting by Oliver Holmes and Dominic Evans in Beirut and Suleiman al-Khalidi in Amman; Writing by Douglas Hamilton; Editing by Mark Heinrich)