Activists say Islamic State militants are advancing toward the historic city of Palmyra in central Syria, seizing an oil field and taking control of the water company on the outskirts.

The advances in the Palmyra countryside were coupled with an announcement Saturday by the Islamic State group that its fighters have seized full control of Saker Island in the Euphrates River north of Deir el-Zour, the northeastern province divided between areas held by the group and the government.

The new gain, confirmed by the Britain-based Syria Observatory for Human Rights, leaves the Deir el-Zour military air base in the clear path of Islamic State fighters. It follows days of intense clashes.

Government forces and Islamic State fighters began battling on the eastern edge of Palmyra on Thursday.