Al-Qaeda-affiliated militants and other Syrian rebel groups have attacked the government-held city of Idlib, briefly seizing several official buildings.
The Nusra Front said its fighters killed dozens of government soldiers before being pushed back.
The Syrian government has maintained control of Idlib city in the country's north-west since it was briefly taken over by rebel groups in 2012.
Syria's civil war, in its fourth year, has claimed more than 200,000 lives.
In other developments, a video believed to have been produced by Islamic State (IS) militants purports to show the abducted British journalist John Cantlie in the Syrian Kurdish town of Kobane.
In the video, Mr Cantlie refers to events from the last fortnight, including a US air drop of weapons and ammunition intended for Kurdish fighters in Kobane.
The US has been carrying out air strikes on Kobane to help Syrian Kurdish forces repel an IS advance.
Intense fighting has been reported in and around the town, which is close to the Turkish border.
Turkey last week said it would allow Iraqi Kurdish forces to enter the town to support the Syrian Kurdish fighters. However, the deployment has yet to materialise, the AFP news agency reports.
Mustafa Qader, a senior Iraqi Kurdish official quoted by the agency, suggested that Turkey was responsible for the delay.
"We are awaiting the stance of the state of Turkey and because of this have not sent any forces," he is quoted as saying.
Turkey, faced with a long insurgency by its own Kurds, has until recently barred access for Kurdish fighters to Syria.
IS seized a broad swathe of borderless territory across Iraq and Syria this summer.
Monday's attack on Idlib began at dawn with rebel fighters attacking the town from all sides, according to the Syrian Observatory for Human Rights (SOHR), a UK-based group that gathers reports from a network of activists in Syria.
The rebels attacked checkpoints and briefly entered the governor's office and a police headquarters before being repelled.
The SOHR quoted an activist as saying that most of the attacks took place on the southern edge of the city, near Mastoumeh Hill.
The rebels reportedly captured the hill, prompting the government to respond with helicopter gunships. Dozens of rebels and soldiers are said to have been killed.
Much of Idlib province is under rebel control but the city that shares its name has remained in the hands of forces loyal to the government of President Bashar al-Assad.
The BBC's Jim Muir in Beirut says the Nusra Front and its allies have been making slow advances in the north-west and south of Syria, while global attention has been focused on the US air campaign against Islamic State (IS).
The SOHR says the Syrian air force has carried out 600 air strikes in the past week, including barrel bombs dropped from helicopters. About 180 civilians have died in the attacks, the group says.
President Assad's government has been battling against an armed and increasingly fragmented uprising. As well as fighting the government, rebel groups such as the Nusra Front and IS have also been fighting among themselves.