Troops fired from helicopter gunships on several neighborhoods of Syria's second city Aleppo on Friday, as the army faced off against rebel fighters, activists told Agence France Presse.

The Syrian Observatory for Human Rights said the army was using helicopter gunships in the southwest of the city, in the Salaheddin, Bustan al-Qasr, Sukari, al-Mashhad and al-Azamiya neighborhoods.

U.N. chief peacekeeper Herve Ladsous said on Thursday there is "no plan B" for Syria, urging all parties to implement the plan brokered by U.N.-Arab League envoy Kofi Annan to stop the deadly violence.

Speaking to reporters as fighting raged across Syria between government troops and rebels, Ladsous said "everything should be done to reduce and put an end to the violence and that includes, of course, the use of heavy weapons by official forces."

Fighting raged in Syria's second city Aleppo on Thursday afternoon, a watchdog said, as President Bashar Assad issued a law establishing a court for terrorism-linked cases.

A security source told Agence France Presse troops were preparing to launch an all-out offensive on rebel-held districts of Aleppo.

Syrian dissidents meeting in Rome signed a joint appeal Thursday for a political solution to the Syria conflict, calling for a ceasefire, the release of detainees and national reconciliation.

"We cannot accept Syria being transformed into a theatre of regional and international conflict," said the 17 signatories of the appeal, including leaders of the National Coordination Body and the Democratic Forum.

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.