An opposition activist group and a Kurdish official say fighting is taking place in a northern Syrian town held by the Islamic State group after the militants did not respond to an offer to withdraw from the encircled town within 48 hours.

The Britain-based Syrian Observatory for Human Rights is reporting fighting in the town of Manbij Friday.

Members of the predominantly Kurdish U.S.-backed Syria Democratic Forces have been on the offensive in Manbij for weeks, backed by U.S.-led coalition airstrikes.

On Thursday, the Manbij Military Council — part of the SDF — said IS fighters had 48 hours to leave the town with their "individual weapons," saying this was their last opportunity to leave alive.

Sherfan Darwish of the SDF says the extremists did not respond to the offer.