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 Sunday dawn."
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.