Syrian rebels have clashed with government troops after converging on a military airport in Aleppo, a monitoring group and state media said, as the United States said it was considering "all possible options" to help the opposition.
Buoyed by victory in the strategic town of Qusayr on the border with Lebanon, President Bashar al-Assad's troops have been readying to open a northern front in Aleppo province on the border with Turkey.
The 26-month conflict pitting mainly Sunni Muslim rebels against Assad's regime, dominated by his Alawite sect of Shiite Islam, has on occasion spilled over into Lebanon and Turkey.
It has also threatened to draw in Israel, where a minister admitted on Monday that Assad could triumph in the war which has already killed more than 94,000 and forced millions to flee their homes.
After suffering a string of battlefield losses in and around Qusayr in the past week, rebel fighters advanced on the Minnigh airbase in Aleppo province on Monday, a monitoring group reported.
"Opposition fighters have seized the radar tower in the Minnigh airbase," Syrian Observatory for Human Rights director Rami Abdel Rahman told AFP.
"Fierce clashes have raged in the airbase since dawn Sunday."
State media said the rebel assault had been repulsed.
"Troops from our heroic army stopped terrorist groups from assaulting the Minnigh army airbase," said the official SANA news agency.
The development came as US Secretary of State John Kerry postponed a visit to the Middle East in order to attend White House talks on Syria, US officials told AFP.
Kerry is struggling to put together a peace conference on Syria in tandem with his Russian counterpart Sergei Lavrov, as Washington comes under increasing pressure to arm the opposition, although it has provided aid for things like night-vision goggles and body armour.
Amid wrangling between Syrian opposition leaders and a fierce debate over exactly who should attend, the date for such talks initially slated for May has now slipped to July at the earliest.
President Barack Obama has asked his national security team -- which includes Kerry -- "to consider all possible options that would accomplish our objectives of helping the Syrian opposition serve the essential needs of the Syrian people and hastening a political transition to a post-Assad Syria", National Security Council spokeswoman Bernadette Meehan told AFP.
"We have prepared a wide range of options for the president's consideration, and internal meetings to discuss the situation in Syria are routine," she said.
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But she said there were "no new announcements at this time".
Israel's intelligence minister, Yuval Steinitz, said Assad might succeed in crushing the armed uprising with the help of his main regional ally Iran and the Lebanese Shiite movement Hezbollah.
"It might be the case that, at the end of the day, Assad... might get the upper hand," he said.
"In such time of conflict, if the opposition is not making any progress, and the regime manages to survive and to get very strong support from other countries, namely Iran and Hezbollah, which is a proxy of Iran, in the end it might just survive," he said.
Syria was receiving "very significant militant support" from Iran and Hezbollah, with thousands of Shiite militants fighting alongside Assad's forces in "very clear formations and with very good equipment -- this might help," he said.
Israel has repeatedly shied away from involvement in Syria's civil war, insisting it is not backing one side or the other in the conflict.
Meanwhile Saudi Arabia, the Middle East's main Sunni powerhouse and regional rival of Shiite-ruled Iran, condemned Hezbollah's "flagrant" intervention in the conflict.
Damascus accuses the conservative Gulf kingdom and neighbouring Qatar of arming Syria's rebels.
The monarchies of the Gulf Cooperation Council (GCC) -- made up of Saudi Arabia, Bahrain, the United Arab Emirates, Kuwait, Oman and Qatar -- on Monday announced sanctions against Hezbollah members in response to their armed intervention in Syria.
Under the sanctions Hezbollah members can will face travel and financial restraints.
The oil-rich members of the GCC denounced in a statement the "flagrant intervention of the Lebanese Hezbollah in the Syrian crisis".
The latest violence across Syria killed at least 119 people on Monday, said the Observatory, which relies on a network of activists, doctors and lawyers for its reports.
And in a new sign of the growing savagery of the conflict, an Al-Qaeda front group executed a 14-year-old boy it accused of blasphemy.
The execution of Mohammad Qattaa in Aleppo province on Sunday drew condemnation from the mainstream opposition.
"Anyone who breaches treaties and international conventions, and commits any war crimes will stand trial, regardless of who they are," the National Coalition said.