U.S.-backed forces are pictured near the village of Susah in the eastern province of Deir al-Zour, near the Syrian border with Iraq on Sept. 13, 2018. (AFP photo)

DAMASCUS, Oct. 3 (Xinhua) -- A U.S. aircraft on Wednesday brought Marine forces to the battle against the last pocket of the Islamic State (IS) in Syria's eastern province of Deir al-Zour, a war monitor reported.

The Syrian Observatory for Human Rights said the aircraft landed in the vicinity of the town of Hajin, one of the last IS-held towns in the eastern countryside of Deir al-Zour.

It said the Marine forces will participate in the battle alongside the Syrian Democratic Forces (SDF) against IS militants.

According to the London-based watchdog, the U.S. move aims to intensify the anti-IS military campaign on the eastern bank of Euphrates River.

Since the battle was launched on Sept. 10, the IS militants have been well fortified with landmines and suicide bombings, leading to slow advance of the Kurdish-led SDF backed by the U.S.-led airstrikes.

The pro-rebel monitor also said 211 IS militants, along with 113 SDF fighters and allied militiamen, have been killed in the fighting since Sept. 10.

The Syrian army has cleared the IS from large swathes of areas in Deir al-Zour, while the SDF is attempting to consolidate positions in the northern and northeastern countryside of the province.

Recent reports said the IS-controlled Syrian territory has shrunk to one percent from almost half of the country a couple of years ago.

The Syrian government has slammed these operations in Syria as "illegal" since the U.S.-led coalition entered the Syrian war in 2014, saying the United States has been trying to establish presence in eastern Syria through supporting the Kurdish-led SDF, in order to secure its interests in the country.