BEIRUT (AP) — Islamic State militants are stiffening their defenses for a possible assault on their de facto capital of Raqqa, as international airstrikes intensify on the Syrian city in retaliation for the Paris attacks. IS fighters are hiding in civilian neighborhoods and preventing anyone from fleeing, former residents say.

The northern Syrian city’s estimated 350,000 people are gripped by fear, rattled by powerful Russian and French airstrikes that shake the city daily. They are also worried they would be trapped with nowhere to go amid signs of a looming ground invasion by US-allied Kurdish and Arab forces in Syria, according to the former residents who have fled to Turkey and now report on events in Raqqa through acquaintances and activists inside.

For months, the anti-IS forces have been advancing gradually toward Raqqa with backing from American-led airstrikes, capturing IS-held towns to the north and east of the city. After IS claimed responsibility for Friday’s carnage in Paris that killed at least 129 people, there are calls for even stronger action in Syria.

Iraqi intelligence officials this week told The Associated Press that the operation was planned in Raqqa, where the attackers were trained specifically for this operation with the intention of sending them to France. The attacks came soon after IS claimed the downing of a Russian plane in Egypt and deadly suicide bombings in Lebanon and Turkey.

US Secretary of State John Kerry on Tuesday suggested Raqqa would be the new focus.

“My sense is that everybody understands that with Lebanon’s attacks, with what’s happened in Egypt, with Ankara, Turkey, with the attacks now in Paris, we have to step up our efforts to hit them at the core where they’re planning these things,” he said after his meeting with French President Francois Hollande on Tuesday.

This undated file image posted on a militant website on Jan. 14, 2014 shows Islamic State fighters marching in Raqqa, Syria. (Photo by AP Photo/Militant Website, File)

This undated file image posted on a militant website on Jan. 14, 2014 shows Islamic State fighters marching in Raqqa, Syria. (Photo by AP Photo/Militant Website, File)

But the extremists are digging in to make any potential assault as grueling as possible. The city, which they have held since early 2014, lies on the Euphrates River at an intersection of major routes from all directions, most through agricultural areas crisscrossed by canals and tributaries of the river. The closest forces from the US-backed Kurdish-Arab coalition called the Democratic Forces of Syria are 50 kilometers (30 miles) to the north in the town of Ein Issa.

The Raqqa activists say the militants have been stepping up defenses of the city since late October, after the Democratic Forces launched their campaign vowing to retake the city. Shortly afterward, IS banned people leaving the city and activists said it has stepped up enforcement of the ban in the past few days, leading to fears the group intends to use civilians as human shields in future fighting.

To avoid being hit in their bases, the fighters have moved into residential neighborhoods in empty homes abandoned by people who fled Raqqa earlier, said an activist from Raqqa. He spoke on condition he be identified only by the name he uses in his political activism, Khaled, for security reasons.

“There is major fear in the city, especially with Daesh preventing civilians from leaving the city,” Khaled said, using the Arabic acronym for the group.

Khaled, who now lives in Turkey, is in touch with people back in Raqqa. Raqqa residents could not be reached because of an IS ban on private Internet access across the city.

Among new measures that have been put in place by IS, according to several of the activists, is an order that IS fighters move only in alleys and side streets to avoid detection from the air and not use vehicles at night.

Those measures have intensified after a series of successful hits by the coalition that killed a number of IS leaders, including the Islamic State militant known as “Jihadi John” who appeared in several videos depicting the beheadings of US and Western hostages.

In this file photo released May 14, 2015 by a militant website, a member of the Islamic State group's vice police known as "Hisba," right, reads a verdict handed down by an Islamic court sentencing many they accused of adultery to lashing, in Raqqa City, Syria. (Photo by AP Photo/Militant Website, File)

In this file photo released May 14, 2015 by a militant website, a member of the Islamic State group’s vice police known as “Hisba,” right, reads a verdict handed down by an Islamic court sentencing many they accused of adultery to lashing, in Raqqa City, Syria. (Photo by AP Photo/Militant Website, File)

On the roads leading into Raqqa, the extremists have dug extensive tunnels and trenches, said another activist from Raqqa, who spoke on condition of anonymity for fear of IS retaliation. More recently, the militants placed tires filled with fuel on empty barrels around the city, with plans to ignite them in case of an attack to cloud the skies with smoke.

The Britain-based Syrian Observatory for Human Rights, a Syrian group that closely monitors the war in Syria, said on Wednesday that airstrikes on Raqqa and its outskirts over the past three days killed at least 33 IS fighters.

Last week’s capture by Kurdish forces of the Iraqi town of Sinjar near the Syrian border cut off one main route connecting Raqqa to IS holdings in Iraq, making movement of fighters and supplies more difficult. On the Syrian side, fighters of the Democratic Forces have been on offensive for the past two weeks in Hassakeh province, northeast of Raqqa. Last week they seized the Syrian town of Hol from IS, further crimping its supply lines.

Those forces are now marching south toward the town of Shaddadeh, an IS stronghold 150 kilometers (90 miles) east across the desert from Raqqa, Kurdish activist Mustafa Bali said. Once that is taken, they will head east toward Raqqa through the Abdul-Aziz Mountain, as well as from Ein Issa and Soureen to the north and northwest of Raqqa.

He said liberating Raqqa would be a major blow and “mark the beginning of the end of Daesh in Syria,” and he called for greater international support for the Kurdish-Arab coalition. But a campaign on Raqqa by the under-armed forces would be costly, even with an intensified air campaign, he said.

“I believe it is going to be a major and long war.”