The U.N. Security Council opened negotiations Thursday over dueling Russian and U.S.-backed resolutions aimed at breaking the diplomatic gridlock over Syria as violence continued to rack the country.

Unlike previous disagreements during the 16-month-old Syrian conflict, the current dispute must be resolved before the mandate for the U.N. monitoring mission in Syria expires at the end of next week.

The United States backs a resolution introduced by Britain that would extend the mission with a caveat: Harsh new sanctions would take effect immediately if a cease-fire did not take hold within 10 days and Syrian President Bashar al-Assad’s government forces did not withdraw heavy weapons from population areas. The sanctions would come under Chapter 7 of the U.N. Charter.

Russia’s competing resolution would extend the mission without the sanctions threat. Chapter 7 is a “red line,” Russia’s deputy U.N. ambassador, Igor Pankin, said as talks got underway in New York. “Anything can be negotiated, but we do not negotiate this,” he said.

Three hundred monitors were sent under a previous resolution to check compliance with a cease-fire that was only briefly observed by the government and rebel forces. The monitors remain in Syria, but their operations were suspended last month as the violence escalated.

As closed-door discussions began in New York, the Syrian government said it had fired its ambassador to Iraq, who publicly announced his defection to opposition forces Wednesday. Without mentioning the defection, a Foreign Ministry statement said that Ambassador Nawaf al-Fares had “made press statements that contradict the duties of his position . . . [and] left the headquarters of the Syrian Embassy in Baghdad without having obtained prior consent from the Ministry.”

U.S. officials hailed Fares’s action as evidence of what they hope is an incipient flood of high-level splits within the Assad government.

“We do see the pressure building,” Secretary of State Hillary Rodham Clinton said in Cambodia, where she is attending a meeting of Southeast Asian governments. “Senior military figures from the Syrian army are defecting every week.”

“The economy is in shambles,” Clinton said. “The regime is struggling to hold on to large parts of the country.”

Despite the downward spiral, the international community is still searching for new ways to pressure the Assad government while stopping short of direct intervention.

The new U.S.-backed resolution is designed to speed up the painfully slow diplomatic effort to stop the violence and persuade Russia to abandon Assad and support efforts to install a new unity government.

The U.S. argument to Russia, said a senior Obama administration official who spoke on the condition of anonymity about the ongoing negotiations, is: “How can you possibly want to be in support of a regime that’s shelling Damascus today? Why not do whatever it takes to get the [monitoring] mandate renewed. . . . Help us ratchet up the pressure to do what even you say should be done.”

The cease-fire call was adopted months ago by the Security Council, including Russia, but has had little effect.

In a briefing for council members Wednesday, U.N. special envoy Kofi Annan asked them to impose “consequences” for noncompliance with the cease-fire and other elements of his peace plan, including formation of a post-Assad transition government.

Under complicated U.N. rules, separate paragraphs of the charter’s Chapter 7 authorize members to intervene in ways “not involving the use of armed force,” and to “take such action by air, sea, or land forces as may be necessary to maintain or restore international peace and security.”

The U.S.-backed resolution specifies that “if the Syrian authorities have not fully complied . . . within ten days” of passage, the Security Council “shall impose immediately measures under Article 41,” the nonmilitary paragraph. Sanctions could include international travel bans, asset freezes and arms embargoes that are now imposed only by the United States, the European Union and a few others.

But Russia has indicated that it considers any invocation of Chapter 7 a foot in the door to eventual military intervention.

In laying out the diplomatic battle lines, Clinton said, “We do look to the Security Council and all of its members, including Russia, to join us in a serious resolution” giving Annan “what he needs, what he’s asking for, and imposes real consequences on the regime for continuing to defy its obligations first and foremost to its own people and then to the international community.”

As the deadline approaches for renewing the monitoring mission, it was unclear whether the United States and its allies were bluffing about letting the mission lapse if Russia does not agree to their sanctions resolution. U.N. Secretary General Ban Ki-moon has recommended that the mission remain in Syria so it can “flexibly” respond to possible improvements on the ground.

In a news conference Wednesday, Susan E. Rice, the U.S. ambassador to the United Nations, said that unless the Security Council agrees to “concrete measures to increase the pressure” on Assad, “it’s not plausible to assume [the mission] will be any more able to fulfill its mandate in the future than it is now.”