International mediator Kofi Annan has voiced concern over the shelling of the Syrian opposition stronghold of Homs and reports of the use of mortar bombs, helicopters and tanks in Haffeh, near the coast.

Mr Annan's spokesman, Ahmad Fawzi, said in a statement: "There are indications that a large number of civilians are trapped in these towns.

Mr Annan called for UN military observers, who are monitoring a tattered ceasefire declared two months ago, to be allowed into the town in the Mediterranean province of Latakia.

The statement said: "The Joint Special Envoy demands the parties take all steps to ensure that civilians are not harmed, and further demands that entry of the UN Military Observers be allowed to the town of Haffa immediately."

Mr Annan held talks in Damascus with Syrian President Bashar al Assad two weeks ago in a bid to halt the spiralling violence and implement his six-point peace plan. The Assad government blames foreign-backed terrorists for the bloodshed.

Syrian forces shelled opposition strongholds in the central province of Homs and eastern Deir al Zor yesterday and clashed with rebels in violence that killed 29 people across the country, the Syrian Observatory for Human Rights said.

The Britain-based Syrian Observatory for Human Rights and the Local Co-ordination Committees also reported government shelling in the southern region of Daraa, the northern province of Aleppo, suburbs of the capital Damascus and Deir el Zour in the east. The Observatory said government forces used helicopters in their attacks on Rastan, which is held by the rebels.

Violence has intensified recently despite an internationally-brokered ceasefire that took effect on April 12. The truce has been repeatedly violated.

The aerial assault targeted the strategic river crossing town of Rastan, which has resisted repeated government offensives for months, they said.

"The regime is now using helicopters more after its ground troops suffered major losses," said Rami Abdul-Rahman of the Syrian Observatory for Human Rights. "Dozens of vehicles have been destroyed or damaged" since the end of May.

Syria's Foreign Ministry spokesman Jihad Makdissi recently said rebels are now using sophisticated anti-tank missiles. Videos posted by activists over the past week have shown many destroyed tanks and armoured personnel carriers.

The Observatory reported the deaths of four civilians and an army defector in shelling in the area of Ashara in Deir el Zour, and said another eight unidentified bodies had been discovered nearby. It reported three dead in the Hama shelling.

Russia said its Foreign Minister, Sergey Lavrov, will visit Iran on Wednesday. Russia and Iran are Mr Assad's strongest allies.

Russia and China have vetoed two Security Council resolutions that threatened possible sanctions against Syria.

Russia has insisted it is "speaking for the strict observance of the norms and principles of international law in the interest of supporting regional stability and security."