Iran has accused the United States of helping Islamic State militants in Afghanistan to fuel regional terrorism and threaten neighboring countries.
Visiting Iranian Foreign Minister Mohammad Javad Zarif made the allegations while speaking to a gathering of diplomats, scholars and journalists in Islamabad.
He said U.S. helicopters were found transporting IS members from the eastern Afghan district of Haska Meyna, also called Deh Bala, to unknown locations.
"We see intelligence, as well as eyewitness accounts, that Daesh fighters, terrorists, were airlifted from battle zones, rescued from battle zones, including recently from the prison of Haska [Meyna]," said Zarif, using the Arabic acronym for IS.
Iran and Russia consistently allege that "unmarked" helicopters have been flying to Afghan areas where IS militants are entrenched.
"This time, it wasn't unmarked helicopters. They were American helicopters, taking Daesh out of Haska prison. Where did they take them? Now, we don't know where they took them, but we see the outcome. We see more and more violence in Pakistan, more and more violence in Afghanistan, taking a sectarian flavor," Zarif said.
Washington denies the allegations as "mere rumors" and cites its relentless airstrikes in support of Afghan forces against IS bases in eastern Nangarhar province, where Haska Meyna is located.
American troops also routinely accompany the Afghan forces into battle against IS. Last year, the highest number of U.S. combat casualties anywhere in the world occurred in Nangarhar.
The terrorist group uses Nangarhar as its main base to launch attacks elsewhere in Afghanistan. Lately, IS militants have made territorial gains in the northern Jowzjan province next to the border with central Asian states, raising alarms in Moscow.
At an international conference Kabul hosted late last month, the Afghan national security adviser, Haneef Atmar, offered Russian and Iranian delegates joint investigations into allegations of unmarked helicopters flying IS fighters to battle zones in the country.
Iranian Foreign Minister Zarif said Iran was ready to use its influence, along with Afghanistan's neighboring countries, including Pakistan, to promote peace talks between Kabul and the Taliban to prevent IS from spreading its activities in the region.
U.S. officials accuse Iran of supporting the Taliban, while Afghans have long alleged the insurgent group uses sanctuaries in Pakistan to sustain its violent campaign in Afghanistan.