ISIS militants kidnapped 20 foreigners working at a Libyan hospital, then released them -- under the condition, if they want to live, that they stay put so they can treat members of the Islamist extremist group, a hospital official said. About 30 gunmen tied to the group calling itself the Islamic State stormed Ibn Sina Hospital in Sirte on Monday while a bus was waiting to take the workers to Tripoli, Libya's capital. The medical workers were later released and sent back to their homes near the medical facility, a hospital official said Tuesday. But they can't go far, with ISIS militants ordering them not to leave Sirte, according to the official. One of those kidnapped, a doctor from Uzbekistan, was told he is safe as long as he didn't leave, and he treated any militants who were wounded, the hospital official said. "They told him that, for your life, you (stay) and work in the city," the official added. Like the doctor, the other medical workers aren't Libyan. Most are from the Philippines, with others from Ukraine, India and Serbia. The kidnappings came days after people of Filipino, Austrian, Czech, Ghanaian and Bangladeshi descent were taken from Libya's Al-Ghani oil field, an operation that Libya's internationally recognized government blamed on "ISIS militias." The abductions are more evidence of the turmoil Libya has experienced since 2011, the start of an uprising against longtime dictator Moammar Gadhafi. Sirte was Gadhafi's hometown, where he was killed, and where his loyalists held out the longest. In the years since, Sirte has become a home to ISIS. The coastal city, halfway between Tripoli and Benghazi, is where terrorists kidnapped 21 Coptic Christians from Egypt in separate incidents in December and January. That mass abduction was followed by mass slaughter -- one that, in ISIS' distinctive, depraved style, was videotaped and disseminated as propaganda, showing jihadists standing behind their orange-clad, handcuffed victims and then beheading them. "The sea you have hidden Sheikh Osama bin Laden's body in," a masked English-speaking jihadi said on the video, "we swear to Allah, we will mix it with your blood." Like the Egyptians, the medical workers kidnapped Monday are foreigners, having come to Sirte to work. They also have skills that could help ISIS. The Ibn Sina Hospital official believes the extremist group went after the workers -- who'd been trying to flee Sirte's precarious security situation -- because they make up the only medical team there and might be needed to treat ISIS militants.