Russia and China vetoed on Saturday a Western-Arab UN Security Council resolution backing an Arab League call for Syrian President Bashar Assad to step aside.

The other 13 council members voted in favor of the resolution, which would have said that the council "fully supports" the Arab League plan.

Mohammed Loulichki, the UN ambassador of Morocco, the sole Arab member of the 15-nation council, voiced his "great regret and disappointment" that Moscow and Beijing joined forces to strike down the resolution.

French Ambassador Gerard Araud told the council, "It is a sad day for this council, a sad day for all Syrians, and a sad day for democracy."

Diplomats said that China had been expected to follow Russia's lead. Russia's decision tovote against the resolution came after U.S. and European officials rejected a series of Russian amendments to the draft resolution.

The Syrian opposition said Saturday that the veto by Russia and China against the United Nations Security Council resolution on Syria was disappointing.

"This veto came at the expenses of the Syrian people and their blood," Naji Tayyara, the head of foreign affairs at the opposition National Syrian Council, told dpa.

"I think the Syrian regime knew in advance that Russia was going to veto the resolution and that is why the regime committed the massacre in (the province of) Homs," he added.

The unusual weekend session came as Syrian forces pummeled the city of Homs with mortars and artillery in what activists called one of the bloodiest episodes of the uprising, with at least 200 people killed.

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. President Barack Obama, meanwhile, issued a stark warning to Assad on Saturday, saying his "relentless brutality" must be stopped and urged him 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 killingand crimes against his own people now. He must step aside and allow a democratic transition to proceed immediately," he said.

In response, the United Nations Secretary General Ban Ki-moon said on Saturday that the UNSC's inability to agree on a resolution to end the violence in Syria undermined the role of the United Nations.

"This is a great disappointment to the people of Syria and the Middle East, and to all supporters of democracy and human rights," Ban said in a statement.

"It undermines the role of the United Nations and the international community in this period when the Syrian authorities must hear a unified voice calling for an immediate end to its violence against the Syrian people."

Ban said that as Syria's crisis deepens, "the Security Council has lost an opportunity to take unified action that could help ... forge a peaceful future, with democracy and dignity," and stressed that the regime must end all violence.

Human rights organizations also responded to the veto on Saturday. Amnesty International said the double by Russia and China was a "betrayal" of protesters.

Meanwhile, the New York-based Human Rights Watch (HRW) said, "Today, after weeks of Russian diplomatic games-playing and in the middle of a bloodbath in Homs, they (vetoes) are simply incendiary ... the risk is high that the (al-)Assad regime will see this double veto as a green light for even more violence."