Police clashed with an armed "terrorist group" in Kumanovo in northern Macedonia on Saturday and parts of the town were sealed off, authorities said.

Macedonia's state-run news agency MIA reported that four police officers were injured by gunfire, and three of them were taken to a hospital in the capital, Skopje.

It later said about 20 injured had been transferred to Skopje hospitals, but didn't specify whether they were police officers, members of the armed group or civilians.

Interior Ministry spokesman Ivo Kotevski refused to give more details to reporters about possible casualties in the incident, referring to the operation as being ongoing.

He said that a "well-trained terrorist group" had entered Macedonia from an unspecified neighboring country with a plan to "perform attacks on state institutions." He said the group was "sheltered in the houses of supporters" in the Kumanovo neighborhood Diva Naselba, but didn't give any more details about the organization.

Police launched a search operation in that area early Saturday morning and Kotevski said "terrorists" attacked special police forces with automatic guns, snipers and bombs.

"This is a risky operation because it is an area with narrow streets and police need to search house to house very carefully", Kotevski said.

Kumanovo is an ethnically mixed town located about 40 kilometers (25 miles) northeast of the capital Skopje, near the border with Kosovo and Serbia. The region was the center of hostilities between ethnic Albanian rebels and government forces during the ethnic conflict in 2001.

Ethnic Albanians, who make up a quarter of Macedonia's 2 million people, took up arms in 2001 demanding more minority rights. The conflict ended after six months with a western brokered peace deal that granted more rights to the ethnic Albanian minority.

The EU delegation in Macedonia on Saturday appealed for calm and said in a statement they are waiting "for facts to be established by the relevant authorities."

Serbia, Macedonia's northern neighbor, reacted by sending reinforcements of special police to the border region, apparently fearing a possible spillover of tensions.

The incident came a day after thousands of opposition supporters joined nationwide protests against alleged police brutality in Macedonia. The protests started after opposition leader Zoran Zaev — citing illegally recorded conversations — accused the government of trying to cover up the 2011 police killing of a 22-year-old man.

The recordings are part of a series of wiretaps Zaev has been releasing amid Macedonia's most severe political crisis in years.

Zaev on Saturday appealed for calm, but has earlier called for a large anti-government protest on May 17.

The junior coalition partner in the conservative government, the ethnic Albanian Democratic Union for Integrations, or DUI, has also expressed concern and appealed for calm. DUI urged people not to respond to provocations.

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Testorides reported from Skopje. Jovana Gec in Belgrade, Serbia, contributed to this report