Islamic State is thought to control about a third of Iraq and Syria. (AP)

Islamic State militants have launched an attack on a major military air base in eastern Syria that is the last significant government outpost in the extremist-dominated region, activists said.

The airfield, just outside the city of Deir el-Zour, is a key military facility for President Bashar Assad, giving his planes a hub from which to bomb IS-held cities and towns across much of eastern Syria.

For the Islamic State group, capturing the airport would eliminate the remaining pocket of resistance in the area and provide a major morale and propaganda boost after a string of setbacks.

The assault began under the cover of darkness with a suicide car bombing against a Syrian military position on the airfield's outskirts, the Britain-based Syrian Observatory for Human Rights said.

The Local Coordination Committees, an activist collective, also reported the fighting. Both groups said heavy clashes raged amid government shelling on the villages surrounding the airfield.

The Observatory, which relies on a network of activists on the ground, said at least 19 government troops and seven IS militants were killed in the fighting.

If the attack does mark the start of a major push to capture the air base, the Syrian government may be forced to decide how much to invest in holding onto the outpost in territory otherwise dominated by the jihadi group.

Earlier this summer, IS militants captured a series of Syrian government military bases outside the north-east city of Raqqa, giving them full control of the surrounding province.

The extremists killed hundreds of Syrian troops captured in those battles, shooting many in mass killings, while beheading others and parading their bodies through IS-held towns and cities.

The government came under unusually heavy criticism from its supporters following those losses.

PA Media