Syrian tanks shelled the central city of Hama and parts of Homs came under mortar fire on Tuesday, opposition activists said, on the day President Bashar al-Assad had agreed to halt the use of heavy weapons and withdraw forces from urban areas.

Tanks were still present in both cities, activists said.

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 fire striking neighbourhoods in the centre and east of Homs, the hub of a 13-month-old uprising.

In Hama, Manhal Abu Bakr reported hearing shelling 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 over Skype. Internet video, which Abu Bakr said was filmed in Hama overnight, showed nighttime explosions in a built-up district.

The Syrian Observatory for Human Rights said most cities were relatively calm on Tuesday after heavy bloodshed in recent days, but reported no clear sign of troop withdrawals.

There were no immediate reports of action by fighters of 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 the southern city 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; Editing by Douglas Hamilton and Mark Heinrich)