A suicide car bombing at a Syrian military intelligence headquarters in Damascus province killed 53 people, a watchdog said on Friday.

The January 24 attack, reported for the first time by the Britain-based Syrian Observatory for Human Rights, was carried out by the jihadist rebel group the al-Nusra Front, the watchdog said.

The head of the International Committee of the Red Cross called on all sides of the Syrian conflict to help his organization access more of the war-ravaged country with desperately needed aid.

Peter Maurer told Agence France Presse in an interview that his organization had distributed food and other urgent aid items to more than 1.5 million people last year, and had helped 17 million Syrians access safe water.

Some 420,000 people -- half of them children -- in the hard-hit Syrian region of Homs are in desperate need of humanitarian aid, the U.N.'s children's agency said Friday.

"These people need life-saving assistance," UNICEF spokeswoman Marixie Mercado told Agence France Presse, pointing out that many of the 420,000 people most in need of help were living in "shelters with blasted-in windows."

Israeli warplanes flew over southern Lebanon Friday, two days after an airstrike near Damascus, as Syria's army chief of staff warned against testing his country's capabilities.

Gen. Ali Abdullah Ayoub made his comments Thursday during a visit to some military units in the country. The al-Baath newspaper, the mouthpiece of President Bashar Assad's ruling party, quoted Ayoub as saying Syria will never change its stance "no matter how much the enemy carries out provocative and hostile acts."

Israeli media on Friday warned that an alleged air strike on a convoy carrying arms from Syria to Hizbullah could set off a chain reaction, and reported troops on high alert in the country's north.

There was still no official Israeli comment on Syrian claims that Israeli warplanes bombed a military site near Damascus on Wednesday or on separate reports that its aircraft struck a weapons convoy along the Syria-Lebanon border.

Southern Damascus saw fresh clashes early on Friday, as army tanks battered rebel enclaves near the Syrian capital, a monitoring group said.

In northern Syria, unidentified gunmen opened fire on a bus in Aleppo province, killing four civilians, among them three university students, said the Syrian Observatory for Human Rights.

U.S. Secretary of State Hillary Clinton accused on Thursday Iran and Hizbullah of playing an increasingly prominent role in the Syrian war.

The U.S. is "disturbed by increasing Iranian and Lebanese Hizbullah activities" in Syria, Clinton told reporters on the eve of her last day as secretary of state.

International Syria mediator Lakhdar Brahimi said Thursday he has no plan to return to Damascus and gave a guarded response to an offer by an opposition leader for talks with government figures.

"It is worthy of note," Brahimi said of a statement by Syrian National Council leader Moaz al-Khatib that he was "ready for direct discussions" outside of Syria.

The opposition Syrian National Coalition said on Thursday that any talks on the country's political future must be about the departure of the regime of President Bashar Assad.

A day after SNC chief Moaz al-Khatib expressed openness to discussion with members of the regime, the political commission issued a statement reaffirming the group's charter that "any negotiation or dialogue must be about the departure of the regime and its pillars."

U.N. leader Ban Ki-moon is gravely concerned about reports of an Israeli air strike on Syria but cannot independently verify what happened, a spokesman said Thursday.

On top of a protest to the United Nations, Syria has also complained to the U.N. Security Council about the raid, diplomats said.