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 the Pashto language. For the next 90 minutes, speakers deliver sermons about 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 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 eastern region, a rich agricultural area and strategic trade corridor. Fighters loyal to the Islamic State, known here by its Arabic acronym Daesh, are reportedly reaching as close as 12 miles from this provincial capital as they wrest control of areas where Afghan security forces largely remain confined to outposts.

Islamic State forces in Afghanistan have been mostly a mixture of disaffected Taliban and tribal militants from Pakistan and of Uzbek and Chechen fighters. Recently, though, 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 that the militants and the new radio station are being sponsored by Pakistan, which officials in that country deny.

A market in Jalalabad, Afghanistan, where women wear all-covering burqas outside their homes. Residents are terrified that fighters from the Islamic State are coming close to the city. (Pam Constable /The Washington Post)

Provincial leaders said they are frustrated with the poor results of a recent military offensive that failed to push back Islamic State militants. With NATO air power no longer available to provide cover to Afghan troops, they said, the government is being forced to rely on poorly trained local police and even on Taliban fighters — widely viewed here as a lesser, homegrown evil — to take on the better armed and financed Islamic State forces.

“The Afghan soldiers fight well. But they are badly managed, and the government is weak. How else can 20 Daesh fighters capture districts where they have sent in 2,000 army troops?” demanded Ahmed Ali Hazrat, the provincial council head. He said that local armed tribes are ready to take on the Islamic State in coordination with the army but that their offer has been rebuffed. “Without international air support, there is no way the Afghan forces can defeat them,” he said.

Last week, Gen. John Campbell, the top U.S. and NATO military commander in Afghanistan, signaled growing U.S. concern about the Islamic State’s ambitions here, saying that its leaders may be trying to create a regional stronghold in Nangahar. On Friday, Defense Secretary Ashton B. Carter briefly visited a U.S. base in Jalalabad, where he predicted a “hard” year ahead in the fight against the Islamic State and the Taliban.

Leaders from four districts, including two close to Jalalabad, said their areas are now partly or fully under Islamic State domination. An elder from Chaparhar, a short drive from Jalalabad, said the extremists are “beheading four to five people a day” there and control 90 percent of the district. He said that one family was murdered in recent days and that Islamic State fighters returned two of their severed heads to local authorities. The elder, who fled with his family to Jalalabad, spoke on the condition that he not be identified.

“They just kill people; they don’t say why,” he said. “They have already warned me. If they see my name, they will kill me, too.”

‘Everyone is afraid’

The launching of Caliphate Radio has magnified the intimidating power of the Islamic State and has made the group seem more ubiquitous. Officials said the broadcast can be heard throughout most of the province. In Jalalabad on Saturday evening, every word came through clearly.

“This is an infidel government, with an infidel system,” one speaker said. “Those who are in the same trench with infidels are not Muslim” and are therefore worthy of death. He included in that group anyone who delivers goods to government agencies, shares offices with foreigners, or befriends Jews or Christians. “It is negative propaganda that we kill everyone,” he asserted, “but we are fighting to finish an evil system.”

The chilling effect is palpable in this bustling city of 350,000, which is full of hotels and restaurants that cater to tradesmen and travelers. On a recent day, the streets emptied quickly after dark, and police and army vehicles circulated. Business owners said trade was down; officials said the city was flooded with jobless men who had fled conflict zones.

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

“These people are not human. They have much money and many big weapons. They can put Dashikas on every hilltop,” he said, referring to Russian-made DShK heavy machine guns. He spoke on the condition of anonymity, fearing for his security.

“They easily slip in and out across the border, and they bring everything in on horses. We wonder who is really behind them,” he said.

The radio is worrisome for a second reason: its potential to appeal to young Afghans who are alienated, idle or already influenced by radical Islamist teachings. Local officials say the area’s high unemployment rate makes young men susceptible to the Islamic State’s promises of high pay.

Another group of potential recruits are students at Nangahar University, the second largest in the country. In November, Islamist student activists staged an ­anti-government protest, waving both Taliban and Islamic State flags. Police made 27 arrests.

One economics instructor, who spoke on the condition of anonymity because of security concerns, said the campus — once known as a center for medical and technical studies — is now so full of radicalized students that it is too dangerous for a foreigner to visit. In some dormitories, he said, students are watching extreme Islamist videos and becoming swept up in the idea of violent jihad.

A Sunni cleric here, Maulvi Zahir Haqqani, who is working with the government to counter the Islamic State’s message, said he and his fellow scholars issued a fatwa, or religious edict, this week denouncing the militia’s predations as un-Islamic and encouraging residents to defend themselves against the “invaders.”

Haqqani said the Taliban, though cruel, is more palatable here because it consists of fellow Afghans, ethnic Pashtuns and Hanafi Muslims, a mainstream Sunni sect. The Islamic State is largely made up of Salafist Muslims, who follow an ultraconservative strain of the faith that originated in Saudi Arabia and Egypt, and the group is led by Middle Easterners.

The Islamic State radio broadcasts told listeners in recent days that Taliban fighters should be higher-priority targets than the government. Relations between the two Islamist militias are complex, with some Taliban members joining the Islamic State while others are fighting it. Afghans worry that their growing armed rivalry will sow chaos in the region.

Already, elders and officials from across Nangahar said, the conflict has emptied Achin, close to the Pakistani border, and has sent residents fleeing from six more of the province’s 23 districts — Khogyani, Chaparhar, Deh Bala, Shinwar, Behsud and Pachiragam. In the past week, they said, Islamic State fighters have reached an area of Chaparhar that is 12 miles from Jalalabad.

“The women are all locking themselves in their houses because they have heard about the flags Daesh is demanding,” said Habiba Kakar Qazizada, a teacher from Behsud. Islamic State fighters have ordered that flags be raised over houses with widows or unmarried daughters, whom they claim the right to take as spoils of war. “The Taliban treat women like goods, too, but they are more lenient,” she said.

The broadcasts of Caliphate Radio have made it clear that the insurgents do not plan to stop until they control Afghanistan. On Friday, before signing off with a stirring, familiar melange of ancient and modern battle sounds, the announcer vowed that “soon our black flags will fly” over the presidential palace in Kabul.

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