US President Barack Obama and French President Nicolas Sarkozy called on Syria to abide fully with a UN-backed plan to stop a "brutal crackdown" and vowed to intensify efforts to end the violence, including steps at the UN Security Council.

In a statement issued after a conference call between the two men, the leaders also urged Iran to undertake serious negotiations when major powers begin talks with the Islamic Republic in Istanbul on Friday.

"The two presidents urged the Syrian regime to respect scrupulously and without preconditions its commitments to the plan of the Arab League-UN envoy. The regime will be judged by its acts," the statement said.

UN-Arab League envoy Kofi Annan told the Security Council on Thursday that Syria has not fully complied with the terms of his peace plan and urged the 15-nation body to demand the withdrawal of troops and heavy weapons from towns, council diplomats said.

Paris previously has said that it would go back to the Security Council if Damascus failed to implement the six-point plan that includes a political transition.

"With their partners, notably Arabs, they (Sarkozy, Obama) agreed to intensify their efforts to finally end the brutal repression of the Syrian people so that an immediate humanitarian assistance can be given... Those who have committed acts of violence will have to answer to their crimes," the statement said.

White House spokesman Jay Carney told a news briefing late Thursday that "the leaders called on the regime to fully implement the Annan plan, noting that the international community would judge the regime by its actions, and not its words."

He also said that Syrian forces were currently observing a "tentative" not full ceasefire, and that by keeping forces in place in cities, Syrian President Bashar Assad was still in a position to resume assaults against the opposition. It would not be a full ceasefire until all troops and heavy equipment are pulled out of urban areas, Carney said.

Later Thursday, US Secretary of State Clinton expanded on Obama's Syria policy, saying that the ceasefire was just a first step and that the United States supported sending an advance team to Syria immediately ahead of a potential UN monitoring mission.

Clinton said the "Group of Eight" foreign ministers welcomed that violence in Syria had abated, but that this was just one element in former UN Secretary-General Kofi Annan's plan. She said Syria must meet other obligations, including a pullback of troops and tanks from cities and allowing humanitarian aid.

"The United States supports sending an advance team immediately to begin this work," Clinton told reporters at the State Department.

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The advance group as well as the UN monitoring mission "will need complete freedom of movement, unimpeded communications, and access throughout the country and to all Syrians as well as firm security guarantees from all parties," she said.

Jpost.com staff contributed to this report.

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