JALALABAD, Afghanistan (Reuters) - Islamic State militants, under pressure from Afghan and U.S. forces, have seized a new stronghold in Tora Bora, a mountainous area dotted with caves along Afghanistan’s border with Pakistan, officials said on Thursday.

The remote region in Nangarhar province was most famously used by al Qaeda leader Osama bin Laden in late 2001, in a bid to hold out against the U.S. troops and Afghan allies who toppled the Taliban regime.

Now Afghan officials say Islamic State militants have seized cave complexes in Tora Bora, after days of fighting against Taliban who had been based there.

“Those areas around Tora Bora were a Taliban stronghold, but now Daesh militants captured them during fighting,” the police commander in the area, Shah Wali, told Reuters, using an Arabic term for Islamic State.

Taliban spokesman Zabihullah Mujahid acknowledged that Islamic State forces had managed to capture several villages, but he denied that they had seized Tora Bora.

General John Nicholson, the top U.S. commander in Afghanistan, has vowed to defeat Islamic State there this year, and in April used one of the largest conventional bombs ever dropped in combat to target a cave complex used by Islamic State in the nearby district of Achin.

“After Achin, Daesh was looking for a second stronghold and now they have it,” Wali said.

Abu Omar Khorasani, an Islamic State commander in Afghanistan, told Reuters that his fighters had seized Tora Bora and were also battling government troops, who are backed by U.S. ground troops and aircraft.

“We are in Tora Bora but this is not the end,” Khorasani said. “The plan is to take more territory from the government and the Taliban.”

The fighting has sent hundreds of families fleeing, said Malek Tor, a tribal elder who put the number of Islamic State fighters in the area in the hundreds.

An official with the U.S. military command in Kabul said Islamic State forces are “on the run” and “are attempting to take refuge” in the Tora Bora region.

“No matter where they are, there is no safe haven for them in Afghanistan,” the official said in a statement. “We will continue toward our goal of defeating ISIS-K in Afghanistan this year and ending their barbaric campaign of death, torture and violence against the Afghan people.”

Government forces have launched new operations targeting Islamic State, but more fighters are being recruited or crossing the border from Pakistan, said Attahullah Khogyani, spokesman for the Nangarhar governor’s office.

“You kill one Daesh fighter and 10 more come from the border or are recruited here,” he said.