JALALABAD, Afghanistan - At exactly 6 p.m. across this nervous city and surrounding districts, a clandestine radio broadcast comes to life each night with sounds of clashing swords, drumming hoofbeats, and bursts of machine-gun fire.

"Caliphate Radio, where hell welcomes the conspirators of infidels," intones the announcer in Pashto. For the next 90 minutes, speakers deliver sermons on Islam, recite Koranic verses in Arabic, threaten death for anyone connected with the "infidel" government and call on young Afghans to join their holy war.

No one is sure where the week-old broadcasts are coming from. Officials say that they are attempting to track the radio broadcast facility and silence it, but they suspect it is mounted on a truck, moving among the tribal regions that straddle the nearby border with Pakistan. The program can be heard throughout Nangahar province but not nationally.

Already, the broadcasts have struck new fear into residents of this besieged region, a rich agricultural area and strategic trade corridor. Fighters loyal to the Islamic State, known here as Daesh, its Arabic acronym, are reportedly arriving as close as 12 miles from this provincial capital as they wrest control of areas where Afghan security forces remain largely confined to outposts.

Islamic State's forces in Afghanistan have been mostly a mixture of disaffected Taliban and tribal militants from Pakistan, plus Uzbek and Chechen fighters. Recently, however, there have been indications that some fighters from the Middle East have joined them and are attempting to establish a stronghold in Nangahar. Many Afghans suspect the militants and the new radio station are being sponsored by Pakistan, which Pakistani officials deny.

"Everyone is afraid," said one council member from Achin, where Islamic State fighters remain in control four months after they executed villagers by forcing them to sit on explosives.

A Sunni cleric here, Maulvi Zahir Haqqani,said the Taliban, though cruel, are more palatable here because they are Hanafi Muslims, a mainstream Sunni sect. The Islamic State is largely made up of Salafist Muslims.