US 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, and called on Assad to cede power."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.After activists reported that more than 200 people were killed in shelling by government troops in Homs, Obama 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.The day of violence in Homs was the bloodiest of an 11-month uprising and it gave new urgency to a push by the Arab League, the United States and a UN resolution calling for Assad to cede power.But Russia has resisted the resolution, saying the Security Council must avoid taking sides in a civil war."The council now has an opportunity to stand against the Assad regime's relentless brutality and to demonstrate that it is a credible advocate for the universal rights that are written into the UN Charter," Obama said.US Ambassador to the United Nations Susan Rice said on Saturday that proposed Russian amendments to the resolution could not be accepted."They are unacceptable," Rice told reporters as she went into closed-door council consultations on Syria.The council had originally scheduled an open meeting for Saturday to vote on the draft. But Russia, still dissatisfied with the text, requested that the 15-nation body not immediately do so and instead hold consultations.Western diplomats said they were still looking for a vote on Saturday on the draft."We are determined to vote today," France's UN Ambassador Gerard Araud told reporters.The changes proposed by Russia, seen by Reuters, would introduce language assigning blame to Syria's opposition, as well as the government, for violence in which the United Nations says more than 5,000 people have died.Russian Foreign Minister Sergei Lavrov said that consensus is possible on the resolution if Security Council members take a "constructive approach" to Russia's proposed changes, Itar-Tass news agency reported."Our amendments do not demand any extreme efforts," Lavrov said upon return to Moscow from Munich, where he met with US Secretary of State Hillary Clinton hours ahead of a planned vote on a draft resolution Russia opposes."If our colleagues display a constructive approach, we will get ... a collective Security Council resolution that I am certain all countries without exception will sign onto," he was quoted as saying.Western nations reject the idea of equal blame, saying the government is mainly responsible.Araud said the existing text clearly supported the Arab League plan. "We are not going to move from that," he said, adding that the Russians "just want to gain time."It was not immediately clear how long the closed consultations would last.Western diplomats said they did not know whether Russia would vote for the draft, abstain or veto it if it came to a vote on Saturday.