04/05/2012 16:02 GMT

UNITED NATIONS, April 5, 2012 (AFP) - International envoy Kofi Annan said Thursday that "alarming" casualties were being reported in Syria despite the government's claims of a partial troop withdrawal from protest cities.

The UN-Arab League envoy insisted however that the government and opposition groups should aim for a complete cessation of hostilities by April 12.

"I urge the government and the opposition commanders to issue clear instructions so that the message reaches across the country down to the fighter and soldier at the local level," Annan told the UN General Assembly.

Annan's briefing to UN envoys and a new statement by the UN Security Council, which Russia and China agreed to, added international pressure on President Bashar al-Assad over his assault on opposition forces.

The Syrian government has agreed to withdraw troops and heavy weapons from cities by April 10. Annan said that if the government keeps its commitment he would call for a complete halt to hostilities by "0600 hours Damascus time on Thursday April 12."

Western governments say they have strong doubts that Assad will comply.

Annan said Syria had told him it had started a partial withdrawal from Idlib, Zabadani and Daraa, but added that he had no confirmation.

"It is clear that more far-reaching action is urgently required. Immediate and verifiable steps are needed to complete implementation of commitments in the crucial days ahead," he said.

"Clearly the violence is still continuing. Alarming levels of casualties and other abuses continue to be reported daily," he added.

"We must silence the tanks, helicopters, mortars, guns and stop all other forms of violence too: sexual abuse, torture, executions, abductions, destruction of homes, forced displacement and other abuses including on children."

Annan said opposition groups that his team had spoken to had "committed to call for cessation of violence once the Syrian government has demonstrably fulfilled its commitments."

The international envoy held talks in Damascus with Assad last month and the government has accepted his six-point peace plan, which includes the withdrawal of troops and weapons as steps to launching a political end to the conflict.

Annan told the UN General Assembly that the first "steps must implemented in a way that sends a powerful political signal of peace."

Just before Annan spoke, the UN Security Council agreed a statement that called on Syria to "urgently" move to keep an April 10 deadline to withdraw troops and weapons.

Russia, Syria's last major ally, and China agreed the statement giving important new backing to the special envoy by formally approving the April 10 deadline.

The Security Council statement "calls upon the Syrian government to implement urgently and visibly its commitments" made to Annan to take the steps toward a cessation of hostilities.

It calls for Syria to start a two hour daily pause in hostilities and to allow immediate humanitarian access.

The council said that depending on Annan's reports on what Assad has carried out, it will "consider further steps as appropriate."

The Security Council has been badly divided on Syria. And the new statement was softened at Russia's demand, diplomats involved in the talks said. Russia and China have vetoed two previous full resolutions on Syria.

An initial proposal by western countries that the council "demands" that Syria pull back its troops and heavy weapons was changed to "calls" and "verifiably" was changed to "visibly."

Diplomats said that the council could face a new battle if Annan says that Syria has failed to keep the agreement and calls for UN sanctions. Russia and China are unlikely to agree any tough measures, they said.

tw/ao