(Reuters) - At least 21 people were killed and around 100 wounded overnight when Syrian army planes bombed a town in northern Syria that is controlled by Islamic State militants, a group monitoring the war said.

Syrian military helicopters dropped barrel bombs and warplanes launched air strikes on al-Bab which lies northeast of the city of Aleppo, the Britain-based Syrian Observatory for Human Rights said on Sunday.

One of the 21 killed was a child and the death count was expected to rise as some of the wounded were in a serious condition, said the Observatory, which gathers information from a variety of sources in Syria.

U.S-led warplanes have also been hitting Islamic State targets in Syria in a campaign which Washington says is not coordinated with Damascus. The Syrian army previously hit an area near al-Bab in September, saying it had "eliminated a number of terrorists" shortly after U.S.-led strikes started.

There was no immediate report on the latest strikes on Syrian state media.

Since the U.S.-led forces started strikes on Syria more than a month ago, Syria's military has ramped up its own air raids, concentrating on the west of the country and at times targeting territory held by Western-backed rebel fighters.

The U.S.-led strikes have mainly focused on Syria's north and east where Islamic State and other militant Islamist groups hold territory in areas bordering Turkey and Iraq.

(Reporting by Sylvia Westall; Editing by Andrew Heavens)