PARIS — 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.