BEIRUT (Reuters) - The U.S.-led coalition against Islamic State has for the first time airdropped local ground forces behind enemy lines near the ISIS-held town of Tabqa in northern Syria, the Pentagon said on Wednesday, opening a new front in the campaign to recapture nearby Raqqa city.

The U.S. military had airlifted allied Syrian Arab militia fighters near Tabqa, providing them fire support in a move aimed at retaking a major dam that remains in the hands of Islamic State, also known as ISIS, the Pentagon said at a news conference in Washington.

U.S. troops took part in the operation, at least flying to the site. Colonel Joseph Scrocca, a coalition spokesman, declined to say whether U.S. advisers were on the ground, but left open that possibility.

The Syrian Democratic Forces (SDF), a militia alliance including Arab and Kurdish fighters, said the operation also aimed to block any advance by Syrian government forces from the west.

Islamic State captured the Tabqa Dam, also known as the Euphrates Dam, and a nearby major air base at the height of its expansion in Syria and Iraq in 2014. It is located about 40 km (25 miles) to the west of Raqqa city.

The campaign to capture Raqqa appears to be gathering pace as an overlapping, U.S.-backed effort in Iraq is drawing closer to driving Islamic State from Mosul.

A local official familiar with SDF operations said the airdrop took place on Tuesday.

The SDF, which includes the powerful Kurdish YPG militia, is fighting to encircle Raqqa city with support from the coalition, including air support and backing on the ground from U.S. special forces.

Scrocca said Apache helicopters, Marine artillery, U.S. special operations forces and air strikes were involved in the operation. About 75 to 80 percent of the forces were members of the Syria Arab Coalition, and Kurds were among the remainder, he added.

The operation could take several weeks, Scrocca said. “This is the first time that we have conducted an air movement or an air assault with the Syrian Arab Coalition and the SDF, (I) don’t know if it’ll be the last.”

The landing forces had seized four small villages in the area west of Tabqa and cut a main highway that links the provinces of Raqqa, Deir al-Zor and Aleppo, it said.

PIONEERING STEP

The local official familiar with SDF operations told Reuters the airdrop operation aimed to secure a crossing point for forces coming in boats across the Euphrates, which began arriving at dawn on Wednesday.

The official said the air drop was “a pioneering step.”

The head of the YPG told Reuters last week a final assault to capture Raqqa city from IS would begin at the start of April. The Pentagon said a decision had yet to be taken.

Turkey is pressing Washington for a role in the Raqqa campaign. Ankara is concerned about the growing influence of the YPG, seeing it as an extension of the Kurdistan Workers’ Party (PKK), which has fought a three-decade insurgency in Turkey.

The SDF cut the last main road out of Raqqa earlier this month, narrowing in on the city from the north, east and west.

The only way in or out of Raqqa now is over the Euphrates River that borders the city to the south.

Air strikes by the U.S.-led coalition near Raqqa have escalated this month, the Syrian Observatory for Human Rights said. The Britain-based war monitoring group said an air strike killed at 33 people near Raqqa when it hit a school sheltering displaced people on Monday.

The Observatory said it believed the strike was carried out by the U.S.-led coalition.

Coalition air strikes pounded Tabqa on Wednesday, destroying a bakery and other buildings, the Observatory added. The strikes killed and wounded more than 40 civilians, it said.

Scrocca said the United States had carried out strikes in the area but did not confirm a U.S. strike on the building in question, adding the U.S. military would look into the allegations.

The jihadist group is losing ground to three separate campaigns in northern Syria, by the SDF, the Russian-backed Syrian army and Turkish-backed Syrian rebels.