A convoy of U.S. forces armoured vehicles drive near the village of Yalanli, on the western outskirts of the northern Syrian city of Manbij, on March 5, 2017. (AFP photo)

DAMASCUS, March 22 (Xinhua) -- The U.S.-led anti-terror coalition has airdropped ground forces in northern Syria to aid the Kurdish-led Syria rebels in their offensive on Syria's northern city of Raqqa, the de facto capital of the Islamic State (IS) group, a monitor group reported on Wednesday.

The ground forces were airdropped after midnight Wednesday in the town of Krein, some five kilometers west of the city of Tabaqa, in the countryside of Raqqa, said the Syrian Observatory for Human Rights.

The London-based watchdog group said the U.S. ground forces came to help the Kurdish-led Syrian Democratic Forces (SDF) fighters to cut the road between Raqqa and the northern province of Aleppo, and to isolate Tabaqa from Raqqa city, thus tighten the noose on the IS militants in the two cities.

The operation, the Observatory added, also aims to prevent the Syrian government forces from closing in on Tabaqa, and the military airport under the same name in that area, in case the later succeeded to capture the town of Maskaneh in the eastern countryside of Aleppo, where the Syrian army is making big progress in the battles against the IS group in that part of the country.

The Observatory, which says it relies on a network of activists on ground, said this is the first presence of U.S. forces on the southern bank of the Euphrates River.

The U.S.-led coalition has recently sent forces to help the SDF near the northern city of Manbij in the countryside of Aleppo.

It also airdropped forces near the eastern city of Deir al-Zour last January, killing 25 IS militants.

But regarding Raqqa, the SDF has for months announced an offensive on Raqqa to separate the city from its countryside and from the IS-held areas in the eastern province of Deir al-Zour.

Earlier this month, the Washington Post said several hundred U.S. Marines have been deployed to Raqqa to assist local forces on the ground in a bid to recapture the city.

The newspaper said that the troops will help in the upcoming battle to oust IS from Raqqa, adding that U.S. Marines will be temporarily stationed at "an outpost from which they can fire artillery guns."

Also this month, the SDF said they have enough forces to capture Raqqa from the IS with the support of the U.S.-led coalition.

"The number of our forces is now increasing, particularly from among the people of the area, and we have enough strength to liberate Raqqa with support from the coalition forces," the SDF spokeswoman, Jihan Ahmed, said.

The SDF also said that IS started moving top commanders out of Raqqa, in apparent anticipation of tough confrontation and possibly the city falling out of the IS control.