Afghans claiming to represent a group called the Islamic Organization of Great Afghanistan, which they say is ready to fight for ISIS. Radio Free Afghanistan/ Radio Free Europe Radio Liberty

ISLAMABAD (Reuters) - Pakistan's military has launched a major operation in its volatile tribal areas to stop the Islamic State making inroads into areas bordering Afghanistan, the military's spokesman said on Sunday.

Pakistan has long denied Islamic State has a foothold inside the nuclear-armed nation despite a series of attacks claimed by the group over the past two years, including a bombing in the northern town of Parachinar last month that killed 75.

Military spokesman Lieutenant General Asif Ghafoor said Islamic State — also known as Daesh — was growing in strength inside Afghanistan, prompting Pakistan to launch an operation in Federally Administered Tribal Areas (FATA).

"This operation was necessary because Daesh is getting established there and we have to stop the influence of Daesh spreading into Pakistani territory through the Rajgal valley," Ghafoor said, referring to a valley surrounded by mountains reaching up to 14,000 feet.

He said that the "Khyber 4" operation, which would include the Pakistan air force, would focus on the border areas inside the Khyber Agency area, which is part of FATA.

Ghafoor said across the Khyber border there are safe havens for multiple "terrorist" organizations that are linked to recent attacks in Pakistan, including the Parachinar assault.

Pakistani soldiers stand guard as people who fled the military offensive against militants in North Waziristan, receive food supply from the army in Bannu, in Pakistan's Khyber-Pakhtunkhwa province, July 2, 2014. Thomson Reuters

Northwestern Pakistan's ethnic Pashtun lands are awash with weapons and the area remains the most volatile region in the country despite the military's success in driving out many al Qaeda and Pakistani Taliban militants.

Fighting had subsided in FATA in recent years and many internally displaced people have been sent back to villages reduced to rubble due to military operations against the Islamists over the past 15 years.

"Once this operation is completed, we will first secure the international border on our side and eliminate the hideouts of various terrorist groups," Ghafoor added.

Islamic State has had more success in neighboring Afghanistan, where it controls small chunks of land, but has also faced tough resistance from the U.S.-backed government in Kabul and local Afghan Taliban militants.

Pakistan is seeking support from Afghanistan to control the border but Ghafoor made it clear that it will not allow "foreign boots on the ground" in its territory.

Pakistan's military began building a fence along the 2,611 kilometer border with Afghanistan in May as part of its security program.

(Reporting by Saad Sayeed; editing by Susan Thomas)