At least three gunmen and one security officer have been killed in a nearly 24-hour special operation in the Georgian capital, Tbilisi, as counterterrorism units stormed a multi-storey apartment building on the outskirts of the city.

By 5pm local time (13:00 GMT) on Wednesday, the operation was confirmed completed.

Nino Giorgobiani, deputy head of administration at Georgia's state security service, said later on Wednesday at a news conference that one suspect was arrested during the operation and three others were killed.

One security officer was also killed and four others were wounded, she said.

"According to the information at hand, the criminals were not Georgia's citizens. They had been under surveillance over the last several weeks," she said.

"An investigation is under way within the framework of international anti-terrorism cooperation to confirm their identities and to determine their criminal links."

She did not clarify their nationality, saying more information would be provided in the coming days.

"It is important to note that Georgia's counterterrorism department has been conducting negotiations with them for several hours about surrendering but they rejected it," she said at a previous news conference.

"The members of the criminal group opened fire from an automatic rifle and threw hand grenades."

Government criticised

The Georgian government held a cabinet meeting on Wednesday, but did not make public statements on the security situation.

The prolonged operation prompted criticism of the government from its political rivals.

"Today's events point to the crumbling of the security system in the country. How come these people with such a big stash of weapons that are not even sold in Georgia have gone unnoticed for two years [of their residency in the apartment]," said Nika Rurua, a member of National Movement opposition party, told Al Jazeera in a phone interview.

"It is also very concerning that the special operation lasted 22 hours. 22 hours should not be necessary to kill three people in a country," he said.

"I hope now the incident will be investigated thoroughly and the public will find out who these people were and how so much weaponry was amassed by them."

With reporting by Tamila Varshalomidze