Israel will never defeat Gaza, Khaled Meshaal, the exiled chief of the Hamas movement which rules the Palestinian territory, said in a defiant speech in Sudan on Thursday.

"This enemy is weak and cannot vanquish Gaza," Meshaal told a major conference of Sudanese Islamists.

Russia on Thursday warned nations backing the Syrian opposition against President Bashar Assad that they would violate international law if they started supplying the rebels with weapons.

"Promises are being made from a number of capitals of massive supplies of modern weapons," foreign ministry spokesman Alexander Lukashevich told reporters.

British Foreign Secretary William Hague said Thursday that Hamas "bears principal responsibility" for the surge in deadly Gaza violence, saying it should cease attacks against Israel immediately.

He also strongly urged Israel to do its utmost to reduce tensions, avoid civilian casualties and help create the conditions for peace, warning: "It is imperative to avoid the risk of a spiral of violence."

Iran's foreign ministry on Thursday condemned as "organized terrorism" a vast Israeli operation launched in the Gaza Strip.

Iran "strongly condemns the criminal act of Zionist (Israeli) military forces in killing civilians," ministry spokesman Ramin Mehmanparast said in remarks published on the website of state broadcaster IRIB.

Gunfire from Syria hit the Israeli-occupied Golan Heights on Thursday morning, the Israeli military said, in the latest spillover of violence from the bloody civil war raging across the ceasefire line.

"Stray bullets fired in Syria reached Israel. No soldiers were injured by the gunfire," a spokeswoman said.

Egypt's President Mohammed Morsi rejected on Thursday what he called Israel's aggression in Gaza, saying it threatened to destabilize the region, while Cairo called on the US to stop Israel's strikes.

"The Israelis must understand that we do not accept this aggression, which can only lead to instability in the region," Morsi said in televised remarks, as Israeli air forces pummeled Gaza and militants fired rockets back in a deadly tit for tat.

French President Francois Hollande will on Saturday meet the new head of Syria's new opposition National Coalition, Ahmed Moaz al-Khatib, in Paris, his office said.

France on Tuesday became the first Western country to recognize the coalition as the sole representative of the Syrian people.

Violence in Syria since an uprising against President Bashar Assad's regime erupted 20 months ago has claimed the lives of more than 39,000 people, mostly civilians, a watchdog said on Thursday.

The Syrian Observatory for Human Rights listed 27,410 civilians, 9,800 soldiers and 1,359 military deserters as among those killed since the uprising began on March 15, 2011.

The Damascus region came under artillery fire and air strikes on Thursday as the army launched a major operation, a watchdog said, as activists decried the critical humanitarian situation.

The artillery was focused on the town of Daraya, just southwest of Damascus, the Syrian Observatory for Human Rights said, also reporting shelling on the northeast town of Mudamiyeh al-Sham and air raids on the nearby town of Saqba.

France will raise the issue of excluding defensive weapons from the current arms embargo on Syria to help rebels fighting President Bashar Assad's regime, Foreign Minister Laurent Fabius said on Thursday.

"For the moment, there is an embargo, so there are no arms being delivered from the European side. The issue... will no doubt be raised for defensive arms," Fabius told RTL radio.