Syrian troops fought with rebels near the border with Turkey on Friday, but opposition activists said the brief clash appeared to be an isolated violation as a fragile U.N.-brokered truce entered a second day.

President Bashar Assad's opponents called for widespread protests to test the regime's resolve to comply with the truce, which is at the center of international envoy Kofi Annan's six-point plan to stop the slide toward civil war and launch talks on a political transition.

A 13-month uprising against Assad had become increasingly militarized in response to his brutal crackdown, with an estimated 9,000 people killed. The truce, which formally went into effect at dawn Thursday, was the first internationally brokered cease-fire since the Syria crisis erupted.

Western powers and Syrian opposition leaders remain skeptical the regime will stick to pledges that it will go along with Annan's plan. The Syrian government has broken promises in the past and full compliance could put Assad at risk. Opposition leaders predict that protesters would flood the streets if they no longer have to fear violence and could quickly bring down the president.

The head of the opposition Syrian National Council has called for such mass protests Friday, the day of weekly street marches since the uprising began in March 2011. Under Annan's plan, the Syrian government must allow peaceful protests.

In a sign that the regime might not tolerate large demonstrations, the Interior Ministry warned in a statement carried by the state-run SANA news agency Thursday that demonstrators would have to seek government permission for any marches.

Protesters usually take to the streets after Muslim noon prayers on Fridays. It appeared unlikely the opposition would request permits or that the government would grant them.

U.N. chief Ban Ki-moon said Thursday that it was up to Assad's regime to keep the peace.

"The onus is on the government of Syria to prove that their words will be matched by their deeds at this time," Ban said, warning that "another gunshot" could doom the truce.

On Friday, Syrian troops briefly clashed with opposition fighters on the outskirts of the northwestern village of Khirbet el-Joz that borders Turkey. The army deployed tanks in the area before the clash, according to the Britain-based Syrian Observatory for Human Rights, which has a network of activists throughout Syria.

The tanks were advancing toward a post run by rebel fighters, said another activist group, the Local Coordination Committees, which reported "very heavy gunfire."

Turkey's state-run Anadolu Agency said gunshots could be heard from the village of Uluyol in Hatay province, which is across the border from Khirbet el-Joz. The agency said at least four Syrian tanks were seen in the area.

Rami Abdul-Rahman, head of the Observatory, said the fighting lasted for about half an hour and he had no reports of casualties. "It is all quiet now," he said.

There also were no signs of the widespread shelling or rocket and mortar attacks by regime forces that were daily occurrences before the cease-fire went into effect.

Still, the government has ignored a key provision of Annan's plan to pull troops back to barracks.

The presence of tanks and troops could discourage any large gatherings, but Syrian National Council leader Burhan Ghalioun urged Syrians to demonstrate peacefully. "Tomorrow, like every Friday, the Syrian people are called to demonstrate even more and put the regime in front of its responsibilities _ put the international community in front of its responsibilities," he said on Thursday.

Mass protests would be an important test of whether Assad will allow his forces to hold their fire and risk ushering in a weekslong sit-in or losing control over territory that government forces recently recovered from rebels.

So far, the military crackdown has prevented protesters from recreating the powerful displays of dissent that ushered in the Arab Spring and led to the successful ouster of autocratic leaders in Egypt and Tunisia.

An outbreak of violence at a chaotic rally could give the regime a pretext for ending the truce. And it would be difficult to determine the source of such an attack, given that Syria is largely sealed off from journalists and outside observers.

Annan also has urged the 15-nation U.N. Security Council to authorize an observer mission that would keep the cease-fire going and to demand that Assad order his troops back to barracks, U.N. diplomats said. The council could adopt a resolution on the observers as early as Friday, the diplomats said on condition of anonymity because the meeting was closed.

Western powers, skeptical that Assad will call off the killings, said an end to violence is just the first step.

U.K. Prime Minister David Cameron urged Syria's allies Russia and China to help "tighten the noose" around Assad's regime. Russia and China have blocked strong action against Syria at the Security Council, fearing it would open the door to possible NATO airstrikes like those that helped topple Libya's Moammar Gadhafi.

U.S. Secretary of State Hillary Rodham Clinton noted that Assad failed to comply with key obligations, such as pulling back tanks.

"The burden of fully and visibly meeting all of these obligations continues to rest with the regime," she said. "They cannot pick and choose. For it to be meaningful, this apparent halt in violence must lead to a credible political process and a peaceful, inclusive democratic transition."

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Associated Press writer Selcan Hacaoglu in Ankara, Turkey, contributed to this report.

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Bassem Mroue can be reached on twitter at http://twitter.com/bmroue