IS militants surround key Deir az-Zour airbase in Syria

The Islamic State group has seized control of a key area surrounding a military airbase in eastern Syria's Deir az-Zour city on Monday.

IS gains on Monday saw the militants cut the airport off from other regime-held territories in the city, the Syrian Observatory for Human Rights said, and leading to fears of a worsening humanitarian situation in the besieged territories.

The group approached the airbase and managed to seize control of the areas areas around its perimeter, as IS militants pressed on their brutal assault on the city for a third day.

"The supply route to the airport is cut, and the city's eastern half is cut off from its western half", Observatory head Rami Abdel Rahman told AFP.

IS militants fired a series of missiles in the attack, killing at least four civilians and wounding dozens more.

"The militants struck regime-held areas inside Deir az-Zour, firing artillery shells and mortars and killing four civilians, including a woman," a local source told The New Arab.

A military source said regime forces carried out more than 20 airstrikes on IS positions on Monday.

"IS has resorted to heavy use of infiltrators and huge reinforcements from Raqqa and western parts of Deir az-Zour province," the source said.

Raqqa, to the north, is the de facto capital of the self-styled caliphate IS declared across Syria and Iraq more than two years ago.

IS already controls more than half of Deir Ezzor city and launched a fierce offensive on Saturday to capture remaining regime-held territories, where the UN estimates 100,000 people live.

The Observatory said the assault has killed at least 28 regime forces so far, as well as 40 IS fighters and at least 14 civilians.

Syria's conflict began with protests against Syrian President Bashar al-Assad in 2011, but it has since morphed into a war that has killed 310,000 people and witnessed the rise of radical militants like IS.