Syria’s government has accepted a U.N. plan to forge a political solution to the bloody uprising engulfing the country, the United Nations’ special envoy to Syria, Kofi Annan, said Tuesday, even as continuing violence threatened to spill into neighboring Lebanon.

U.S. officials and others reacted cautiously to President Bashar al-Assad’s latest commitment, citing the Syrian leader’s failure to follow through on previous promises of restraint.

Assad’s pledge “must now be matched by immediate action,” Secretary of State Hillary Rodham Clinton told reporters in Washington.

“We will judge Assad’s sincerity and seriousness by what he does, not by what he says,” Clinton said. “If he is ready to bring this dark chapter in Syria’s history to a close, he could prove it by immediately ordering regime forces to stop firing and begin withdrawing from populated areas.”

The announcement was made in Beijing, where Annan held meetings with Chinese Premier Wen Jiabao and said afterward that his plan had China’s full support. The U.N. Security Council endorsed the plan last week, and Annan said Russia, another strong ally of Syria, had agreed over the weekend to back it.

The six-point proposal calls for Assad to work with Annan in an “inclusive, Syrian-led political process” to address the aspirations of the Syrian people.

It demands an end to the government’s crackdown on the opposition, which launched its protests peacefully but now includes armed elements, and calls for the implementation of an unspecified U.N. mechanism to halt violence on both sides. Other demands include a daily two-hour cease-fire to facilitate humanitarian work, the release of prisoners and access for journalists.

Annan submitted the plan during a visit to Damascus, the Syrian capital, this month. His spokesman, Ahmad Fawzi, said implementation of the proposal “will be key, not only for the Syrian people, who are caught in the middle of this tragedy, but also for the region and the international community as a whole.”

China’s and Russia’s support for the plan may have helped persuade Assad to embrace it. The two countries twice rejected proposed U.N. resolutions criticizing Syria, saying they were not balanced and did not condemn violence by the rebels.

A Paris-based member of the opposition Syrian National Council said it welcomed the Assad government’s decision, the Associated Press reported. “We hope that we can move toward a peace process,” Bassma Kodmani told the wire service.

But other activists greeted the news with skepticism, noting that the Syrian government only partially implemented a previous Arab League-led plan to halt the violence. “It’s like other initiatives that have been before,” a rebel spokesman known as Abu Rami said in the central city of Homs, which has been the scene of fierce fighting between Syrian forces and heavily outgunned protesters. “They did not stop the shelling.”

Indeed, many Syria observers argue that Assad is seeking to bog down Annan and his team of mediators in a fruitless diplomatic process that will provide him with political cover to continue his military campaign against the opposition.

“Assad has everything to gain from accepting the Annan initiative,” said Joshua M. Landis, director of the Center for Middle East Studies at the University of Oklahoma.

Abu Rami said 14 people, including a child, were killed in Homs on Tuesday by sniper and heavy artillery fire. The assertion was difficult to verify because Syria restricts access for journalists. Earlier in the day, Assad toured the Baba Amr neighborhood, a former rebel stronghold that was subdued after a month of military assaults.

Syrian state media said the president had issued a statement promising to restore safety, security and the rule of law.

“We will see now in the days ahead what exactly Assad has said,” Robert Ford, the U.S. ambassador to Syria, told lawmakers in Washington. “I have to tell you that my own experience with him is you want to see steps on the ground and not just take his word at face value.”

Meanwhile, in northern Lebanon, heavy fighting was audible near the border area of Qaa, and some Syrian refugees reported that Syrian soldiers had crossed a few hundred yards into Lebanon in pursuit of armed rebels.

The Lebanese army sealed off the area, blocking access for reporters. Col. Pierre Assaf, a spokesman for the Lebanese army, insisted that the clashes occurred only on the Syrian side of the border, between Syrian troops and members of the Free Syrian Army. One mortar shell fell on the Lebanese side, he said, and no casualties were reported.

Later Tuesday, Assaf said Lebanese soldiers had stopped two buses carrying 10 armed men, five Syrian and five Lebanese, and arrested them.

Syria is expected to top the agenda at a gathering of Arab foreign ministers in Baghdad on Wednesday ahead of an Arab League summit in that city Thursday, and the issue could prove divisive. Although Arab states support Annan’s plan, there is no consensus on what steps should be taken to enforce its implementation.

“It will change the discussion,” said an Arab League official, who was not authorized to speak publicly and spoke on the condition of anonymity. However, the league will first have to read the letter sent by Assad to it and to the United Nations to study exactly what Syria has agreed to, the official said.

Staff writer Colum Lynch in New York, correspondent Liz Sly in Iraq and special correspondent Suzan Haidamous in Qaa, Lebanon, contributed to this report.