An attacker blew himself up after throwing an explosive device into the US embassy compound in Podgorica, the Montenegrin government said Thursday.

Authorities in Podgorica have not released any theories as to the motive for the attack.

"In front of the @USEmbassyMNE building in #Podgorica, #Montenegro an unknown person committed suicide with an explosive device. Immediately before, that person threw an explosive device," the government tweeted, saying the device was "most probably" a hand grenade.

A US State Department spokesperson confirmed "a small explosion near the US Embassy compound" saying officials were "working closely with police to identify the assailant(s)".

A guard at the sports centre who asked not to be named said he "heard two explosions, one after another".

On its Twitter account, the embassy said it had cancelled all visa services for Thursday, adding that access for US citizens "will be available today on an emergency basis".

At 00:30, in front of the @USEmbassyMNE building in #Podgorica, #Montenegro an unknown person committed suicide with an explosive device. Immediately before, that person threw an explosive device from the intersection near the Sport Center into the US Embassy compound. (1 of 2) — Govt. of Montenegro (@MeGovernment) February 22, 2018

New NATO member

The heavily guarded US embassy building is located on the outskirts of Podgorica's city centre, near the secret police headquarters and the Moraca River.

Montenegro, a small Adriatic state of some 660,000 people, joined NATO last May. The decision to become a member provoked violent protests by the pro-Russian opposition in 2015.

In October 2016, authorities said they had thwarted a plot by pro-Russian fighters to storm parliament and overthrow the pro-Western government on the eve of general elections.

Authorities alleged that "Russian state bodies" were involved in the conspiracy, which they say aimed at preventing Montenegro from joining NATO.

In October 2011, the US embassy in Sarajevo in neighbouring Bosnia was attacked by a Muslim man.

Mevlid Jasarevic opened fire with an automatic rifle at the embassy, wounding a police officer. He was also injured in the exchange of fire and arrested.

Jasarevic was later sentenced to 15 years' imprisonment.

According to figures published in November by a regional think-tank, a thousand people from the Western Balkans have gone to join the fighting in Syria and Iraq since 2012.

Twenty-three of those were from Montenegro, whose population is predominantly Orthodox Christian.

Last month, a court in Montenegro for the first time sentenced one of its citizens for having fought in Syria.

Hamid Beharovic, 39, was found guilty of having fought for the Islamic State in Iraq and Levant (ISIL) also known as ISIS, between April 2015 and May 2016. He was given a six-month jail term.