Gunmen took at least 170 people hostage at the Radisson Blu hotel in Mali’s capital Bamako, a hotel spokesman told NBC News early Friday.

“They have locked in about 140 guests and about 30 employees,” a spokesperson for the Carlson Rezidor group, which owns Radisson, told NBC News. “So the hotel is locked down and there is no possibility to go out or come in.”

A security source told Reuters that gunmen attacked the hotel, although NBC News was not able to verify this.

“As per our information the persons who have entered the building and have initiated the hostage-taking have locked the property, so it is not locked by police or other forces,” the spokesperson added.

The chain said it was working to establish the names and nationalities of those taken.

The U.S. Embassy in Mali said it was aware of an “ongoing active-shooter operation at the Radisson Hotel.”

“The U.S. Embassy staff has been asked to shelter in place,” it said. “All U.S. citizens should shelter in place. Private U.S. citizens are encouraged to contact their family. Monitor local media for updates. U.S. citizens should adhere to the instructions of local authorities and monitor local media.”

In August, the Embassy issued a security message informing U.S. citizens “of a heightened security risk to westerners in southern Mali, including the area outside Bamako city.”