next Image 1 of 2

prev Image 2 of 2

Syrian government forces have kept up an escalation against the last rebel-held enclave in northwestern Syria with a wave of airstrikes that killed several people Friday, activists and state media reported. Russia said its air base in a nearby province came under attack.

The increased fighting threatens a cease-fire negotiated by Russia and Turkey, in place since September. The agreement has since been undermined with recurrent cycles of violence.

The Britain-based Syrian Observatory for Human Rights, which monitors the war in Syria, said fighting during the last four days had killed at least 38, calling it the "fiercest escalation" since the cease-fire came into effect.

The Observatory said at least a dozen air raids overnight on the southern edge of Idlib province killed six civilians. On Friday, government warplanes dropped over 68 indiscriminate barrel bombs on several villages, including a dozen in Kfar Nabudah alone.

The activist-operated Shaam news agency put the death toll from Friday at seven and said the airstrikes have 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.

Ratcheting up the tension, a senior Russian official, Gen. Viktor Kupchishin, said the Russian military had foiled an attempted attack on its air base in the nearby coastal province of Latakia. 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 blocked.

The U.N. said it's deeply concerned by the violence. It said the escalation has caused massive displacement inside the enclave that extends between Idlib and Hama provinces and is home to about 3 million people.

"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.