DAMASCUS, Feb. 13 (Xinhua) -- At least 50 fighters of the U.S.-backed Syrian Democratic Forces (SDF) and the Islamic State (IS) group were killed within the past four days of battles in eastern Syria, a war monitor reported on Wednesday.

The SDF and the IS militants are engaged in intense battles in a bid by the SDF to eliminate the IS militants in the eastern Euphrates River region in eastern Syria, according to the Syrian Observatory for Human Rights.

The Britain-based observatory said the IS militants are now besieged in a 2-square-km area in the eastern Euphrates River region.

It said the group rejects to surrender to the U.S.-backed SDF.

The U.S.-led anti-terror coalition is backing the SDF in their push to strip IS of its last areas in the eastern Euphrates.

A day earlier, the state news agency SANA said as many as 70 civilians were killed and wounded by U.S.-led airstrikes on the last IS-held areas.