The United States, France and 13 other nations demanded Thursday that Syria immediately cease military operations against rebel forces and allow unfettered deployment of U.N. observers, suggesting that use of force will be considered if Damascus fails to comply.

The stand, at a French-organized meeting of foreign ministers and other officials from the “Friends of Syria” group, signaled a growing impatience among Western leaders and Arab allies over Syrian President Bashar al-Assad’s delay in putting into practice a six-point peace plan, including a truce, that was negotiated by joint U.N.-Arab League envoy Kofi Annan but has yet to halt the killing.

Despite its implied threat of seeking U.N. authorization for stronger measures, however, the declaration also underlined the difficulty of applying pressure on Syria as long as Russia and China refuse to go along. Both nations wield veto power in the U.N. Security Council, and although they endorsed Annan’s peace plan, they have rejected calls for more forceful action against the Syrian government.

Significantly, Russia and China declined invitations to attend the meeting Thursday.

French Foreign Minister Alain Juppe, who spoke for the group, said a U.N. observer mission in Syria is “at a critical moment” because of “the refusal of Syria to carry out its commitments.” France will soon submit to the Security Council a resolution outlining what Juppe called a “robust” mission that he hoped Russia and China could be persuaded to approve.

Otherwise, he added, the group agreed that it would “envisage other means to resolve this tragedy.”

U.S. Secretary of State Hillary Rodham Clinton, at a briefing, took the warning a step further, saying that if Assad continues to balk at implementing Annan’s peace plan, the United States and its allies should seek a Security Council resolution invoking Chapter 7 of the U.N. charter. That article opens the door to the use of force; it was the basis of the Western intervention in Libya last year to help rebel forces overthrow Moammar Gaddafi.

“I think we have to do more to take stronger action against the Assad regime,” she said. “We need to start moving very urgently in the Security Council for a Chapter 7 sanctions resolution, including travel, financial sanctions, an arms embargo and the pressure that that will give us on the regime to push for compliance with Kofi Annan’s six-point plan.”

Signs of impatience

In the meantime, Juppe said the U.N. observer force should have several hundred members and should be equipped with modern surveillance equipment and independent transportation, even if it is unarmed. Its patrols should be allowed to travel anywhere in Syria without prior notice or government authorization, he said at a news conference.

In another illustration of the impatience with Assad, French President Nicolas Sarkozy accused the Syrian government of planning to destroy the rebellious central city of Homs and said Western powers should set up humanitarian corridors to protect opposition forces.

Sarkozy, who is running for reelection, did not say to what extent — if at all — France would be willing to enforce the corridors. Asked about the president’s statements, Juppe said he and the other ministers had discussed humanitarian aid to rebel forces and displaced Syrian families but not humanitarian corridors.

The French president made his comments as violence continued to flare intermittently across Syria despite the presence of an advance team of U.N. observers. Four civilians were killed Thursday by security forces, according to the Britain-based Syrian Observatory for Human Rights. The group said three people died north of Damascus and one died in the eastern city of Deir al-Zour. It was not possible to verify the toll, which was based on reports from rebels in Syria. Armed opponents of Assad also have continued to attack security forces, according to activists and Syrian state media.

In New York, U.N. Secretary General Ban Ki-moon, concerned about the faltering cease-fire, recommended that the Security Council approve the deployment of 300 unarmed observers to monitor the truce negotiated by Annan.

In a letter to the council, Ban proposed that it establish a full-fledged U.N. monitoring mission of “blue berets” backed by air transport and authorized to carry out unimpeded investigations into any cease-fire violations by the Syrian government or armed opponents.

Ban’s letter to the Security Council said the proposed U.N. Supervision Mission in Syria would be deployed within weeks of the council’s adoption of a resolution creating the body.

Meanwhile, U.S. Defense Secretary Leon E. Panetta told Congress, “It’s clear that the only way the United States would get involved militarily is if there’s a consensus in the international community.”

Responding to questions about Sarkozy’s remarks, Panetta testified to the House Armed Services Committee that “we’ve looked at a variety of options as to what could be done, including the possibility of developing humanitarian corridors. Again, we’re prepared to do whatever the international community ultimately agrees ought to be done.”

Ban’s letter provided a mixed account of the security conditions in Syria since the United Nations deployed its first monitors this week, noting that “it remains a challenge to assess accurately unconfirmed and conflicting reports of developments in Syria.”

Ban wrote that “levels of violence dropped markedly” in Syria after April 12, when the U.N.-brokered cease-fire went into effect. He added, however, that the government has not pulled heavy weapons back as promised and that, according to the government, rebel forces have continued their attacks.

“The plan of Annan is a failure since the beginning,” said Col. ­Malik al-Kurdi, an officer with the rebel Free Syrian Army speaking by telephone from a refugee camp in Turkey. “How can the U.N. send observers after over a year of violence? What are they going to find out, when everything is known to the world?”

Correspondents Alice Fordham in Beirut and staff writers Colum Lynch at the United Nations and Karen DeYoung in Washington contributed to this report.