The increased fighting has undermined the cease-fire negotiated by Russia and Turkey, in place since September. The agreement has since been weakened by recurrent cycles of violence.
The Britain-based Syrian Observatory for Human Rights, which monitors the war in Syria, described the escalation as the "fiercest" since September. It said fighting during the last four days had killed at least 38.
After a wave of airstrikes, rebel groups and the Observatory said government troops and allied fighters attempted to advance on the rebel enclave from the Qalaat al-Madiq area in Hama province, clashing with insurgents there. The Observatory said the attempted advance followed intense airstrikes on the southern edge of Idlib province and northern Hama.
The rebel forces, organized under the name of the National Front for Liberation, said they repelled the advancing government troops and destroyed two vehicles. They also said they had killed government soldiers, a claim that could not be immediately verified.
The activist-operated Shaam news agency said the rebels had killed Syrian soldiers using armed drones.
The head of the Observatory, Rami Abdurrahman, said there had been a buildup of government troops in the area for days. He said the attempted advance appeared designed to expand government control of strategic areas and protect the Russian air base in Latakia. Moscow has previously warned that hostilities in Idlib and the surrounding area threaten the base.
A senior Russian military official based in Syria, Gen. Viktor Kupchishin, said his military had foiled an attempted attack on the Latakia air base Friday. He did not provide further information on the attack. Insurgents in Idlib and Hama have previously used drones to target the base but most of the attacks were thwarted, although they often serve to ratchet up tensions in the area.
Earlier Friday, the Observatory said at least a dozen air raids overnight on the southern edge of Idlib province had killed six civilians. On Friday, government warplanes dropped over 68 indiscriminate barrel bombs on several villages, including a dozen in Kfar Nabudah alone.
Shaam news agency put the death toll from Friday at seven and said the airstrikes also targeted schools in two villages and a medical center in one.
Syrian state media said the airstrikes targeted "terrorist groups," and reported that insurgents had also lobbed missiles into government areas.
The European Union called on Russia and Turkey to "safeguard" the cease-fire. EU spokeswoman Maja Kocijancic said Friday that increased fighting would endanger the lives of the over 3 million people living in the rebel-held areas and further destabilize the country and the region.
The U.N. has also expressed deep concern over the violence. It said the escalation has caused massive displacement inside the enclave, which extends between Idlib and Hama provinces.
"These increased hostilities are triggering large-scale displacement, from northern Hama and southern Idlib. There are also reports of deserted villages after civilians fled for safety. An estimated 323,000 people are estimated to have been displaced in the northwest since September of last year," U.N. spokesman Stephane Dujarric said late Thursday. The U.N. said at least 60 civilians were killed in April.