An unknown man has thrown an explosive device at the United States embassy building in Podgorica, the capital of Montenegro, before blowing himself up, media in the city say.

The attack came five minutes before midnight on Wednesday, web portal portalanalitika.me said, adding that citizens in the area heard two explosions.

“There was an incident,” a police official, who declined to give his name, told Reuters. He did not provide details.

A Reuters photographer in Podgorica said a police vehicle blocked the street where the embassy is located, adding that no damage was visible.

“The US embassy in Podgorica advises US citizens there is an active security situation at the US embassy in Podgorica,” the embassy said on its website. “Avoid the embassy until further notice.”

Montenegro, the smallest of all former Yugoslav republics, was the 29th country to join NATO last May.