Fierce clashes pitted Syrian rebels against troops and fighters loyal to Hizbullah in several villages near Qusayr on the border with Lebanon on Tuesday, a watchdog said.

Meanwhile, a military source told AFP the army expects to seize the town of Qusayr, a rebel stronghold, "within days."

President Michel Suleiman condemned on Tuesday the kidnapping of Syriac Orthodox Archbishop of Aleppo Youhanna Ibrahim and Greek Orthodox Metropolitan of Aleppo and Iskanderun Boulos al-Yaziji in Syria on Monday, demanding their immediate release.

He said: “These actions will not achieve democracy or the goals of the perpetrators.”

President Bashar Assad is using chemical weapons, most likely sarin, against rebel forces in Syria, a senior Israeli army officer told a conference on Tuesday.

"Assad is using chemical weapons in Syria," said Brigadier General Itai Brun, head of research and analysis in the army's military intelligence division, in remarks quoted on the army's official Twitter feed.

Interim opposition chief George Sabra was Tuesday working to secure the release of two bishops kidnapped in a north Syria village, a Greek official said, as a fresh wave of violence hit the country.

On the diplomatic front, U.S. Secretary of State John Kerry was attending his first NATO foreign ministers in Brussels on Tuesday, with Syria the focus, while U.N. leader Ban Ki-moon urged an end to arms supplies to Syria's rival sides.

Two Grad rockets fired from Syrian territories on Tuesday exploded on the outskirts of Lebanon's Hermel town district, causing material damage only.

According to the state-run National News Agency, one of the rockets landed in al-Maali area, damaging and under construction building.

Lebanese Forces leader Samir Geagea condemned on Monday the kidnap of Syriac Orthodox Archbishop of Aleppo Youhanna Ibrahim and Greek Orthodox Metropolitan of Aleppo and Iskanderun Boulos al-Yaziji, describing the incident as "an act of terrorism."

"We promise to contact Arab officials to find, as soon as possible, a solution for this issue,” Geagea told Metropolitan of Aleppo and Iskanderun Boulos al-Yazij over a phone conversation.

Islamist cleric Sheikh Ahmed al-Asir, the imam of Sidon's Bilal bin Rabah Mosque, on Monday announced the creation of the “Free Resistance Brigades,” urging whoever is capable of heading to Syria to go there to aid “the oppressed” in Qusayr and Homs.

Asir called on “all the Lebanese who fear attacks by Iran's party (Hizbullah) in Lebanon to arm themselves and form 5-member secret cells in order to be ready to exclusively defend themselves and their families should the need arise.”

Syriac Orthodox Archbishop of Aleppo Youhanna Ibrahim and Greek Orthodox Metropolitan of Aleppo and Iskanderun Boulos al-Yaziji were kidnapped on Monday at the hands of gunmen near the northern Syrian city, Lebanon's state-run National News Agency reported.

Archbishop Ibrahim picked up Archbishop Yaziji in his car from a village on the Turkish border and his deacon was driving the vehicle, NNA said.

The White House said Monday it was appalled by "horrific" reports of a new massacre in Syria after a watchdog group said over 100 people, many of them civilians, were killed in a town near Damascus.

"We are appalled by horrific reports that hundreds of Syrians were killed over the weekend in a Damascus suburb," White House spokesman Jay Carney said.

The Syrian National Coalition named veteran dissident George Sabra as caretaker leader of the main opposition grouping on Monday, following the resignation of Ahmed Moaz al-Khatib.

Sabra "was assigned today to carry out the functions of the head of the Coalition until elections for a new president," one of the Coalition's main constituent groups, the Syrian National Council, said in a statement.