DAMASCUS  - UN chief Ban Ki-moon called on the Security Council Tuesday to put "sustained pressure" on Syrian President Bashar al-Assad, saying he was "gravely concerned" about the rising death toll, a top aide said.

The head of the United Nations' beleaguered observer mission, which has been struggling to shore up a ceasefire that was supposed to take effect from April 12, was to brief the council on the persistent violence which a human rights watchdog said claimed 36 more lives on Tuesday.

"The secretary general remains gravely concerned about the intensification of violence and rising death toll as well as continued human rights abuses and unmet humanitarian needs," assistant secretary general Oscar Fernandez Taranco said.

"The secretary general has repeatedl.y underscored that the government of Syria bears primary responsibility to change course and fully implement" the peace plan brokered by UN-Arab League envoy Kofi Annan, Taranco said. "A truly joint effort by the council, one that delivers unified and sustained pressure to demand compliance in full with the six-point plan is urgently needed," he added. "Otherwise we may be reaching reaching the day when it will be too late to stop the crisis spiralling out of control."

A Russian ship carrying attack helicopters and missiles destined for Syria was halted off the Scottish coast after its British insurer withdrew cover for the vessel. British Foreign Secretary William Hague said the MV Alaed, owned by Femco, a Russian cargo line, appeared to be head back to Russia after Insurer Standard Club said it had cancelled insurance for the vessel.

"I am pleased that the ship that was reported to be carrying arms to Syria has now turned back apparently towards Russia," Hague told parliament.

The MV Alaed picked up its cargo of Mi-25 helicopter gunships from Russia's Baltic sea port of Kaliningrad, where they had been sent for servicing and repairs, according to Britain's Sunday Telegraph newspaper.