A large explosion outside NATO’s International Security Assistance Force (ISAF) headquarters rocked central Kabul on Thursday, Agence France-Presse reported.

There was no immediate confirmation of casualties, but the blast triggered the U.S. embassy’s “duck and cover” sirens.

According to Reuters, the explosion struck within meters of the entrance to the U.S. embassy in the Afghan capital, police and witnesses said.

A police official said the blast was near a major traffic circle just outside the main entrance to the embassy and the headquarters of the NATO-led ISAF in Kabul.

Reuters witnesses heard alarms blaring across the sprawling compound soon after the blast.

“We can confirm an explosion outside of ISAF HQ around 1020 (am), no other information at this time,” ISAF said on its official Twitter account.

The blast came the day after a Taliban suicide bomber detonated an explosives-laden vehicle near a NATO military convoy entering the city’s airport, killing himself but causing no other deaths or injuries.

“Accident, not blast”

Meanwhile, Afghan officials described the explosion in the arms depot as an “accident,” allaying fears that the attack was by militants.

“It was an accident, not an enemy act,” Lutfullah Mashal, a National Directorate of Security (NDS) official told AFP.

“The blast happened in a depot for unexploded ordinance. It was totally an accident. There is no reported casualties but there are some damages.”