DAMASCUS, Dec. 9 (Xinhua) -- The Kurdish-led Syrian Democratic Forces (SDF) brought in 500 fighters in eastern Syria on Sunday to fight against the Islamic State (IS) in its last stronghold in eastern Syria, a war monitor reported.

With the latest backup, the SDF has amassed 17,000 fighters of allied militias from Kurdish-controlled areas in northern and eastern Syria to take part in the battle against IS, according to the Syrian Observatory for Human Rights.

The latest reinforcement come a day after the SDF and allied Kurdish militias failed to hold on to the positions they had advanced to in the town of Hajin, one of the most important IS-held areas in the eastern Euphrates.

The IS launched a counter-offensive and captured almost all of Hajin after the SDF had become in control of 70 percent of the town in recent days.

The UK-based watchdog group said the fighters have joined the fight Sunday in Hajin amid heavy airstrikes by the U.S.-led coalition, which is backing the Kurdish militias in their push to defeat IS in the eastern countryside of Deir al-Zour province.

On Friday, Syria's official SANA news agency said the intense U.S.-led airstrikes destroyed the only hospital in Hajin and killed eight civilians.

The SDF, with the backing of U.S.-led airstrikes, has launched a campaign to defeat the IS on the eastern bank of the Euphrates River since Sept. 10.

A total of 849 IS militants and 508 SDF fighters have been killed, according to a report released by the Syrian Observatory for Human Rights on Saturday.