Baalbek (Lebanon) (AFP) - Lebanese troops battled militants near the Syrian border Sunday in a second day of clashes that the army chief said left 10 soldiers dead and 13 missing, possibly held hostage.

Militants fired heavy machineguns and mortar rounds at troops in the Arsal area of eastern Lebanon, and sniper fire killed a civilian inside the border town, a security source said.

At a press briefing, army chief General Jean Kahwaji warned that the situation was "extremely dangerous" and described the clashes as a premeditated attack on the military.

"The army has lost 10 martyrs, with 25 more wounded, including four officers, and 13 soldiers are missing, possibly being held prisoner," he said.

A security source, meanwhile, said that a civilian had been killed by sniper fire inside Arsal, raising the civilian death toll in the fighting to three.

The violence is the worst to hit the area since the beginning of the war in neighbouring Syria in 2011.

It erupted after the detention of a Syrian the army said had admitted being a member of Al-Qaeda's Syrian affiliate, Al-Nusra Front.

Kahwaji said the man, Imad Ahmed Jumaa, had been scouting for an attack against the military in the area.

Gunmen angered by Jumaa's arrest surrounded Lebanese army checkpoints on Saturday afternoon before opening fire on troops and storming a police post in Arsal, security sources said.

Two civilians were reported killed, and a security source confirmed that a number of police had been taken hostage, without giving further details.





- 'Decisive and firm' response -

The army said Saturday that it would "not allow any party to transfer the battle from Syria" to Lebanon.

"The army will be decisive and firm in its response and will not remain silent as foreigners try to turn our land into a field for crime and terrorism, murder and kidnapping."

The outbreak of violence sparked clashes in the northern city of Tripoli, where Sunni militants who back the Syrian uprising have regularly fought Lebanese security forces and residents from the Alawite sect who back Syria's President Bashar al-Assad.

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A security source said two soldiers were wounded in Tripoli.

The US State Department condemned the violence and urged all parties to respect Lebanon's policy of "dissociation" from the Syrian conflict.

The US ambassador to Lebanon also met Kahwaji on Sunday to express support, the American embassy in Beirut said.

Prime Minister Tammam Salam condemned the Arsal assault as a "flagrant attack on the Lebanese state and the Lebanese armed forces".

He called on "all political forces to exercise wisdom and responsibility and to make every effort to protect Lebanon and distance it from the dangers around it".

And the Shiite Hezbollah movement, which has dispatched fighters to bolster Assad's forces against an uprising that many in Arsal back, announced its support for the army.

Hezbollah "stands shoulder-to-shoulder with this institution (the army) in the face of threats to our country", the movement said.

Arsal, which currently hosts tens of thousands of Syrian refugees, has frequently been the scene of conflict with Lebanese security forces.

The Sunni-majority area is sympathetic to the uprising against Assad, whose regime has regularly launched air raids in the area that it says target opposition fighters holed up in the mountainous region around Arsal.

The UN refugee agency (UNHCR) told AFP it was "monitoring closely" the situation in Arsal but had no details on whether refugees had been affected by the fighting.





- Influx of refugees -

Tensions skyrocketed in the area earlier this year with a major influx of refugees and fighters after Syrian forces backed by Hezbollah recaptured most of the Qalamun region just across the border.

Despite the Syrian regime's recapture of most of Qalamun, pockets of opposition forces, including jihadists from Al-Nusra and the Islamic State group, remain in the area.

Jihadists engaged in fierce clashes with the regime in the Qalamun region on Friday night, with at least 50 fighters killed, according to a Britain-based monitor.

More than 170,000 people have been killed since Syria's conflict began in March 2011, and the violence has regularly spilled into neighbouring Lebanon, which is hosting more than a million Syrian refugees.