Gunfire and a possible explosion Sunday night rattled the building believed to house the CIA office in Kabul.

It wasn’t clear if the incident took place inside the building or just outside the entrance.

At least one helicopter has landed outside the building, according to local journalists.

The building is in the single most secure part of Kabul and used to be known as the Arianna Hotel until 2002, when it was believed to have been taken over by the CIA. It is just a few feet from one of the entrances to the Presidential Palace, and between two NATO military bases, as well as down the road from the U.S. embassy.

The attack appears to be the third major incident in Kabul in as many weeks — the most violent stretch in the capital in two years. Before the attack began, a U.S. official said an assault two weeks ago on the U.S. embassy and NATO base was likely the “beginning of a trend” of attacks on U.S. installations in Kabul.

The incident began at approximately 8 p.m. local time, when people inside the NATO military headquarters and the U.S. embassy both reported hearing gunshots. The embassy staff was briefly locked down, according to a U.S. official.

Spokesmen for the NATO military coalition in Kabul said they did not have any information about the incident except to say there had been a shooting “in the vicinity of the embassy annex,” believed to be next to the Arianna Hotel.