The U.S.-led coalition against Islamic State (IS) extremists has for the first time airdropped troops behind enemy lines near a strategic Syrian town outside the group's self-declared capital of Raqqa, the Pentagon has said.

The U.S. military airlifted Syrian Arab militia fighters close to Tabqa and provided them with artillery fire support in a move aimed at retaking a major dam that remains in the hands of IS.

The Tabqa Dam, also known as the Euphrates Dam, provides electricity to much of the region and its possible destruction by IS would result in widespread flooding. IS captured the dam and a nearby air base in 2014.

Colonel Joseph Scrocca, a coalition spokesman, said Apache helicopters, Marine artillery, U.S. special operations forces, a few dozen Army rangers, and air strikes were involved in the airdrop operation.

About 75 to 80 percent of the forces were members of the Syria Arab coalition, and Syrian Kurds were among the remainder, he said.

"This is pretty major," Scrocca said, adding that the operation could take several weeks.

"This is the first time that we have conducted an air movement or an air assault with the Syrian Arab coalition.... Don't know if it'll be the last," he said.

The area around Tabqa is a "command and control center for terrorist attacks against the West" and home to numerous foreign-fighter training camps, Scrocca said.

"Seizing Tabqa will give the [coalition] a strategic advantage and a launching point needed for the liberation" of Raqqa, he said. The United States is backing the Syrian Democratic Forces (SDF), a militia alliance including both Arab and Kurdish fighters.

Besides recapturing the dam, SDF said the U.S.-backed operation also aimed to block any advance by Syrian government forces from the west.

The landing forces airdropped into Syria 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, Scrocca said.

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.

While the SDF is fighting to encircle Raqqa with support from the coalition, the militant group is also losing territory it once held to the Russian-backed Syrian Army, Turkish-backed Syrian rebels, and U.S.-backed Iraqi forces in northern Iraq.

Air strikes by the U.S.-led coalition near Raqqa have escalated this month.

Coalition air strikes pounded Tabqa on March 22, destroying a bakery and other buildings, the Syrian Observatory for Human Rights said. The strikes killed or 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.

With reporting by AP, AFP, and Reuters