BEIRUT (AP) — Syrian insurgent groups launched a counteroffensive Thursday against government forces advancing toward a major rebel-held air base in the country's northwest Idlib province, capturing several villages and taking prisoners, opposition groups said.

The push by several factions, including the al-Qaida-linked Levant Liberation Committee, slowed the government offensive toward the Abu Zuhour air base that has been held by rebels since 2015.

Recapturing the air base has been a key government goal since late October and Syrian forces have captured some 160 villages since first launched the offensive. The operations also aim to secure the road linking the capital, Damascus, with Aleppo, Syria's largest city.

The U.N.'s Under-Secretary-General for Humanitarian Affairs Mark Lowcock, in Damascus, said he was "deeply worried" about civilians caught up in the violence in Idlib. The fighting has displaced an estimated 100,000 people who have fled north toward safer areas close to the border with Turkey.

Lowcock, on his first mission to Syria since his appointment last year, called for agreements to allow the U.N. and other relief organizations to reach 2.5 million Syrians in need of aid on a regular basis. They are Syrians in areas the U.N. classifies as "besieged" or "hard-to-reach." The U.N. delivered aid to an average 7.5 million people each month last year.

The under-secretary-general said he was "particularly concerned about the fate of the besieged people of (Eastern) Ghouta," a pocket of the opposition in the suburbs of Damascus. Government forces have waged a punishing aerial and artillery campaign on eastern Ghouta after rebels launched an attack on a nearby military base in November. The bombardment has killed 170 civilians over the last two weeks, according to the Britain-based Syrian Observatory for Human Rights monitoring group.

Attacks by Syrian and Russian forces on eastern Ghouta damaged or destroyed four schools and killed eight children in late October and early November 2017, according to a new report by the New York-based Human Rights Watch. Russia is a key military backer of President Bashar Assad.

"Syrian and Russian forces appear to view the lives of children in Eastern Ghouta as utterly disposable," said Bill Van Esveld, senior children's rights researcher at Human Rights Watch.

Government forces had largely beaten back the counter-offensive by militants in Idlib by evening Thursday, the Observatory reported. The government-controlled Syrian Central Military Media said troops repelled the assault and killed several militants in the fighting.

Idlib is the largest remaining territory in rebel hands, and its population is swollen with more than 1.1 million refugees who have fled fighting from other areas in Syria, according to the U.N.

A statement Thursday by the International Rescue Committee said it received hundreds of newly displaced people from the southeast of the province in the past few days, joining thousands who fled over the past month.

Many of the newly displaced brought only the belongings they could carry, the committee said, adding that one mother of two twin babies recounted the initial panic of fleeing the airstrikes. The woman was so frightened, she initially left one of her children behind.

"We couldn't think properly. The fear affected our brains," the IRC quoted her as saying. The baby was unharmed, she added.

The IRC said nearly two thirds of the displaced in Idlib are living in makeshift tents that are unable to withstand winter conditions while others live in abandoned or partially-build homes that have well water but no toilets.

The Aamaq media arm of the extremist Islamic State group reported Thursday that its fighters are clashing with Syrian troops on the eastern edges of Idlib, and released a video purporting to show four soldiers it claimed to have captured.

The Observatory confirmed insurgents have retaken several villages from government troops and said 11 pro-government fighters were captured. The Observatory's chief Rami Abdurrahman said 16 insurgents were killed in the fighting but didn't provide a figure for government casulaties.

At least 400,000 people have been killed and half of Syria's population displaced since a violent crackdown on anti-government demonstrations in 2011 plunged the country into civil war.

Assad's government says it is fighting a war on terror and describes itself is as a target of a U.S.-led international conspiracy. It says Western sanctions have crippled its economy. Lowcock, in Damascus, said he would like to see "more detailed evidence" about the claims, and said it would be topic of continued discussion with government officials. The U.S. Treasury Department says its sanctions against the Syrian government are in response to human rights abuses and state-sponsored terrorism.

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Associated Press writers Albert Aji in Damascus, Syria contributed to this report.