Concerns about security around Hamid Karzai International Airport in Kabul have increased based on “a very specific threat stream” from ISIS-K about planned attacks against crowds outside the airport, a US defense official has told CNN. The US believes ISIS-K, which is a sworn enemy of the Taliban, wants to create mayhem at the airport and has intelligence streams suggesting it is capable and planning to carry out multiple attacks, according to the official. The US Embassy in Kabul advised US citizens at a number of gates at the airport to “leave immediately,” noting “security threats outside the gates.” “Because of security threats outside the gates of Kabul airport, we are advising U.S. citizens to avoid traveling to the airport and to avoid airport gates at this time unless you receive individual instructions from a U.S. government representative to do so,” the alert said. On Tuesday, as he confirmed his decision not to extend the evacuation deadline beyond August 31, President Joe Biden acknowledged the growing threat the group poses to the airport. “Every day we’re on the ground is another day we know that ISIS-K is seeking to target the airport and attack both US and allied forces and innocent civilians,” Biden said. Concerns increased after more than 100 prison inmates loyal to the ISIS affiliate in Afghanistan escaped from two prisons near Kabul as the Taliban advanced on the Afghan capital, CNN has learned. One regional counter-terrorism source told CNN that as many as several hundred members of ISIS-K may have escaped from the jails at Bagram and Pul-e-Charkhi, both to the east of Kabul. Both prisons fell to the Taliban shortly before they entered Kabul. A Taliban spokesman would not address reports of the escapes when asked by CNN Wednesday, saying only they were not confirmed. As for the risk of terrorist attacks at and around the airport, a Taliban spokesman said Tuesday: “There have been reports that some ill-wishers want to disrupt the security situation there by attacking and harming people and the media. So don’t go close to the airport to avoid being hurt.” According to officials in the previous Afghan government, there were approximately 5,000 inmates at the Bagram facility when the US left it in July. They included members of the Taliban, al Qaeda and ISIS as well as ordinary criminals. ISIS-Khorasan is a branch of the terror group that first emerged in Syria and Iraq. While the affiliates share an ideology and tactics, the depth of their relationship with regards to organization and command and control has never been entirely established. US intelligence officials previously told CNN the ISIS-K membership includes “a small number of veteran jihadists from Syria and other foreign terrorist fighters,” saying that the US had identified 10 to 15 of their top operatives in Afghanistan. The group’s name comes from its terminology for the area that includes Afghanistan and Pakistan: “Khorasan.” The US Defense Department Inspector-General for Afghanistan said in a report covering the months April to June of this year that “ISIS-Khorasan exploited the political instability and rise in violence during the quarter by attacking minority sectarian targets and infrastructure to spread fear and highlight the Afghan government’s inability to provide adequate security.” ISIS-K has formed cells in Kabul, which have carried out a number of devastating suicide attacks in and beyond the Afghan capital since 2016. In May this year, ISIS-K attacked a school for girls in Kabul and killed at least 68 people, wounding more than than 165, most of them girls, according to a Defense Intelligence Agency assessment cited by the inspector general. And in June at least 10 people were killed and 16 others wounded in an attack on the British-American HALO Trust demining charity. ISIS-Khorasan claimed responsibility for the assault on the camp in northeastern Baghlan Province. The group has built up a presence in eastern Afghanistan in recent years, especially in the provinces of Nangahar and Kunar. Last August, the group attacked the main prison in Jalalabad, the capital of Nangahar, in an effort to free dozens of their supporters who had been captured by the Afghan army and police. This story has been updated with the US Embassy warning Americans at certain airport gates to leave.