U.S. President Barack Obama on Saturday urged the UN Security Council to take a stand against what he called the "relentless brutality" of Syrian President Bashar Assad.

After activists reported that 200 people were killed in shelling by government troops in the city of Homs, Obama in a written statement said the attack was an "unspeakable assault" and urged Assad to step down from power.

"Yesterday the Syrian government murdered hundreds of Syrian citizens, including women and children, in Homs through shelling and other indiscriminate violence, and Syrian forces continue to prevent hundreds of injured civilians from seeking medical help," Obama said in a written statement.

"Assad must halt his campaign of killing and crimes against his own people now. He must step aside and allow a democratic transition to proceed immediately," he said.

Security Council ambassadors are gathering Saturday morning to consider a draft resolution backing an Arab League call for Syrian President Bashar Assad to step down.

The unusual weekend session comes as Syrian forces pummel the city of Homs with mortars and artillery in what activists are calling one of the bloodiest episodes of the uprising.

Although Western nations are pressing for a vote, there's no guarantee the resolution will pass.

In a blunt warning to Washington, Russian Foreign Minister Sergei Lavrov said Saturday that if the resolution is put to a vote without taking Russia's opinion into account it will only lead to "another scandal" at the Security Council.

Lavrov said at a security conference in Munich that Moscow still sees two problems of "crucial importance" with the draft resolution on the violence in Syria. Western countries have been working to head off a possible Russian veto if the resolution goes to a vote.

French Foreign Minister Alain Juppe countered that "those who block the adoption of such a resolution are taking a grave historical responsibility" in light of the Homs bloodshed, which he called a "crime against humanity."

U.S. Secretary of State Hillary Clinton had "vigorous" talks with Lavrov in Munich on Saturday about his objections to a UN Security Council draft resolution on Syria and she still hopes Moscow will vote "yes," a U.S. official said.

She and Lavrov agreed on the urgency of the situation in Syria, and Clinton made it clear she believed the vote should go ahead on Saturday as scheduled, said a senior U.S. State Department official, speaking on condition of anonymity.

"The secretary and foreign minister had a very vigorous discussion. They agreed that the situation on the ground is urgent, given the violence," the official told reporters after the meeting took place at the Munich Security Conference.

"The secretary made clear that we feel, that the United States feels strongly that the UN Security Council should vote today," said the official, adding that the "action" now moves to the UN headquarters in New York.

Lavrov also announced on Saturday that he would fly to Damascus this week to meet President Assad, while Syrian opposition activists accused the president of orchestrating a massacre in the central Syrian city of Homs.

Foreign Minister Sergei Lavrov is to leave on Tuesday, he told the news agency Interfax on the sidelines of the Munich Security Conference in Germany. He is to be accompanied by the chief of the Russian foreign intelligence service, Mikhail Fradkov.

Meanwhile, Syrian forces hammered restive neighborhoods in the city of Homs for hours with mortars and artillery Saturday, sending terrified residents fleeing into basements and killing more than 200 people in what appeared to be the bloodiest episode of the nearly 11-month-old uprising, activists said.

The government denied the assault. It said the reports are part of a "hysterical campaign" of incitement by armed groups against Syria, meant to be exploited at the UN Security Council as it prepared to vote.

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