Turkey searched for a missing fighter-jet and held an emergency security summit Friday, as the prime minister backed off reported comments suggesting Syria had downed the aircraft.

The military plane -- reportedly an F-4 Phantom with two pilots aboard -- lost radio contact and vanished off radar screens around 0900 GMT over the eastern Mediterranean, near the border with Syria.

Ankara denied Friday allegations that it is shipping weapons to Syrian rebels across the border, after a report claimed Turkey was among nations arming rebels fighting the regime in Damascus.

"Turkey does not ship weapons to any neighboring country, including Syria," foreign ministry spokesperson Selcuk Unal said when asked whether Turkey was involved in an alleged arms delivery to Syrian rebels.

Syrian troops clashed with rebels in the third-largest city Homs Friday, scuppering a new bid to rescue trapped civilians as the United Nations said up to 1.5 million people were now in need of aid.

The fresh bombardment of the central, Orontes valley city came after at least 168 died on Thursday, the highest single-day death toll since a U.N.-backed ceasefire was supposed to take effect on April 12, a human rights watchdog said. Activists in Homs reached by Agence France Presse via Skype spoke of a "catastrophic situation" in the historic centre and adjacent neighborhoods.

Progressive Socialist Party leader MP Walid Jumblat stressed on Friday that he is still part of the cabinet “until further notice,” confirming his centrist stance.

He slammed some of the March 14-led opposition members for continuously calling on the cabinet to resign.

At least 143 people were killed in violence across Syria on Thursday, among them 100 civilians and 43 government troops, activists and a rights group said.

"This has been one of the bloodiest days in Syria since the anti-regime revolt broke out in March last year," Observatory head Rami Abdel Rahman told Agence France Presse.

U.N. leader Ban Ki-moon discussed the situation in Syria on Thursday with Iran's President Mahmoud Ahmadinejad, a U.N. spokesman said, amid a new diplomatic push to end the crisis.

Ban and Ahmadinejad also spoke about the showdown over Iran's disputed nuclear programs during the talks on the sidelines of the United Nations Conference on Sustainable Development in Rio de Janeiro, said U.N. spokesman Martin Nesirky.

Russian Foreign Minister Sergei Lavrov said on Thursday that any peace plan for Syria that calls on President Bashar al-Assad to leave power and go into exile was not workable because he would not quit.

"A scheme according to which President Assad should leave somewhere before something happens in terms of a cessation of violence and a political process, this scheme does not work simply from the very start," the Interfax news agency quoted Lavrov as saying. "It is infeasible because he will not leave."

Israel's Deputy Foreign Minister Danny Ayalon called Thursday for "massive intervention" by the international community in Syria to prevent the conflict there engulfing neighbors Lebanon and Iraq.

"The longer the wait, the more chaos and victims there will be," Ayalon told a small group of journalists in Paris ahead of talks with French Foreign Minister Laurent Fabius.

Russia on Thursday confirmed for the first time that a cargo ship forced to turn back from British waters was carrying attack helicopters for Syria and said it would now sail under the Russian flag.

Violence has halted the evacuation of trapped civilians from Syria's Homs region, the Red Crescent said on Thursday, adding it was in negotiations for a deal that would allow them to safety.

"Negotiations are still underway with the parties concerned to evacuate civilians, in cooperation with the ICRC (International Committee of the Red Cross)," the Red Crescent's Khaled Erksoussi said.