WASHINGTON -- A months-long campaign to isolate and pressure the Islamic State group's self-declared capital of Raqqa, Syria, is paying off as the administrative backbone of the militant organization is beginning to crack, the Pentagon said Friday.

IS leaders "are beginning the process of leaving Raqqa and moving their operations farther downriver," said Navy Capt. Jeff Davis, a Pentagon spokesman. He said an unspecified number of IS "bureaucrats" are heading east along the north bank of the Euphrates River toward Deir el-Zour, because they see "the end is near in Raqqa."

"We are seeing now an exodus of their leadership," Davis said, adding: "This seems to be a very organized, orderly withdrawal of a lot of their non-combatant support people."

Davis did not predict an imminent collapse of the militant group, and analysts said they expect a tough fight for the Syrian city.

The U.S.-led coalition has been pounding the Raqqa area regularly for months. On Thursday it conducted 17 strikes near the city, targeting two IS military staging areas and an IS combat unit, according to the U.S. Central Command's daily airstrike tally.

It said the attacks destroyed four tunnels, three fighting positions, three IS-held buildings, two weapons storage areas, two IS headquarters, a bridge and other targets. The coalition also launched 11 airstrikes near Deir el-Zour, destroying 20 oil tanker trucks, six oil wellheads, two artillery systems, an oil storage tank and a crane.

"ISIL is going to have to think hard about where they go next. Do they have any place to go?" said Christine Wormuth, the Pentagon's top policy official from 2014 to 2016. Wormuth, now a senior adviser at the Center for Strategic and International Studies, a Washington think-tank , said she expected some fighters would stay in Raqqa and fight.

"The whole point of the isolate mission is to try to squeeze them and get them to leave and flush them out into the open," she said.

President Barack Obama's strategy was to recruit, organize and enable local Syrian Kurdish and Arab fighters to retake Raqqa, rather than put American combat forces in the lead. The Trump administration is now reevaluating that approach and considering options that could include a more direct U.S. combat role.

At his confirmation hearing a month ago, Defence Secretary Jim Mattis said the U.S. approach should be reviewed and "perhaps energized on a more aggressive timeline." He has not said what changes he would recommend.

Last week, the top U.S. commander for the counter-IS campaign in Iraq and Syria, Lt. Gen. Stephen Townsend, said forces leading the fights for Raqqa and the northern Iraqi city of Mosul should prevail within the next six months.

A major complication in the current strategy is Turkey's strong objections to a Syrian Kurd role in the Raqqa campaign. The Turkish government views the U.S.-backed Syrian Kurdish fighters as terrorists and a threat to Turkey. The U.S. sees them as the most effective and reliable element among local fighters supported by the Pentagon.