Syrian troops are massing for a major assault on Aleppo, and hundreds of rebels hunkered down in the strategic northern city's Salaheddin quarter are steeling themselves to fight, and probably to die.

Syria's most populous city, which is also the country's commercial hub, has been rocked by fierce fighting between rebels and the troops of President Bashar Assad for a week now.

Israel on Thursday ramped up security along its ceasefire line with Syria in the occupied Golan Heights as fighting between rebels and President Bashar Assad's regime intensified, Israeli security sources said.

With the clashes spreading across the Syrian side of the strategic plateau, Israeli troops were put on "very high" alert, an Israeli source said, speaking on condition of anonymity.

Syrian President Bashar Assad's regime is bound to topple because of its "abominable behavior", France's foreign minister said Thursday as fighting raged in the cities of Damascus and Aleppo.

"We are continuing our work to end the fighting and for an alternative solution bringing together the Syrian opposition and other actors," Foreign Minister Laurent Fabius said during a visit to Warsaw.

Iran's U.N. envoy on Wednesday accused Israel of staging a suicide bomb attack on an Israeli tourist bus in Bulgaria.

The envoy Mohammad Khazaee said Israel staged the attack, in which five Israelis and their Bulgarian driver were killed, as part of a campaign of "state terrorism operations and assassinations aimed at implicating others for narrow political gains."

Holed up in a Turkish safe-house, a Kurdish commander of a Syrian rebel unit makes a novel pitch for more weapons to help his men fight the regime of President Bashar Assad.

"I wish we could get some armed support from Turkey," said Ubed Muse, speaking to Agence France Presse during a break from the bloody battles in which he has led a band of 45 rebels near Aleppo, Syria's second city.

Tunisian police fired warning shots and tear gas on Thursday to disperse protesters who attacked provincial government headquarters in the town where the country's revolution was born, Agence France Presse reported.

Dozens of people, angry over their living conditions, converged on the building in Sidi Bouzid and set fire to a tire, which they threw inside.

Paris prosecutors on Thursday opened a preliminary probe into the alleged involvement of French firm Qosmos in supplying Syria's regime with surveillance equipment, judicial sources said.

The investigation follows a suit filed by human rights groups against the firm which said that Qosmos may have been supplying equipment that helped President Bashar Assad's regime's bid to crush opposition forces.

Defected Syrian general Manaf Tlass said in comments published Thursday he is working on a plan to end the conflict, save Syria from sectarianism and rebuild the country without Bashar Assad playing a role.

Turkish Prime Minister Recep Tayyip Erdogan has accused Syria of giving Kurdish rebels a free hand in its north and warned that Ankara would not hesitate to strike.

"In the north, it (President Bashar Assad's regime) has allotted five provinces to the Kurds, to the terrorist organization," Erdogan said on Turkish television Wednesday, referring to the Kurdistan Worker's Party (PKK).

Syria's regime confirmed on Thursday the defection of three diplomats, but downplayed its importance and indirectly accused Qatar of encouraging "national division."

The foreign ministry confirmed the defections of Lamia Hariri, charge d'affaires in Cyprus, her husband Abdel Latif al-Dabbagh, ambassador to the United Arab Emirates, and Mohammed Tahsin al-Fakir, security attache in Oman.