The U.S.-led coalition on Wednesday evening foiled a surprise attack by the Islamic State (IS) group in eastern Syria, killing at least 20 militants, a war monitor reported.

The attempted IS attack targeted a military base run by the U.S.-backed Kurdish militia inside the al-Tanak oil field, one of the largest fields in the eastern countryside of Syria's eastern province of Deir al-Zour, according to the Syrian Observatory for Human Rights.

However, the U.S.-led coalition warplanes scrambled and heavily struck the IS positions, destroying seven vehicles and killing 20 militants, said the London-based watchdog.

The heavy strikes succeeded in thwarting the attack, he added.

The oil field contains a base run by Kurdish militia, where the U.S. military advisors and other advisors from the coalition are located.

The IS attack comes as the Syrian Democratic Forces and the Kurdish People's Protection Units were launching a second wave of attack on the last IS-held pocket in eastern Deir al-Zour.

The United States and its allied Kurdish-led forces on the ground aim to clear the eastern Euphrates River of the IS elements as the area is strategically significant due to its proximity to the Iraqi border and its oil-rich fields.

The Syrian government has repeatedly said it will capture all Syrian areas amid negotiations with the Kurdish groups about the future of the Kurdish-controlled region in northern and northeastern Syria.