The EU delegation in Macedonia appealed for calm and said in a statement it is waiting "for facts to be established by the relevant authorities, ABC News reported on Saturday.

Three police officers were killed and at least 20 others were injured in clashes early Saturday between security forces and an armed group in the town Kumanovo in northern Macedonia.

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 had 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.