An armored police vehicle passes near a police check point in Kumanovo, Macedonia May 10, 2015. [Photo/Agencies]

The events will deepen concern in the West over stability in Macedonia, where the government is on the ropes over allegations by the main opposition, the Social Democrats, of illegal wire-tapping and widespread abuse of office.

Protesters demanding the resignation of conservative Prime Minister Nikola Gruevski clashed with police this week and the opposition is threatening to rally thousands on May 17.

Observers fear political leaders on either side may try to stoke ethnic tensions as leverage.

An Interior Ministry spokesman said on Saturday the group in Kumanovo was planning "terrorist acts" and had "infiltrated" the country from a neighbouring state, without saying which.

Macedonia's neighbours are Albania, Greece, Kosovo, Serbia and Bulgaria. Macedonia has previously identified armed men in the country as coming from Kosovo.

An estimated 30 percent of Macedonia's 2 million people are ethnic Albanians. Guerrillas took up arms in 2001, clashing with security forces before the West brokered a peace deal offering the Albanian minority greater rights and representation and the insurgents entered government.

But implementation has been slow and tensions sometimes flare. Many citizens, regardless of ethnicity, are frustrated at the slow pace of development and integration with the West. Macedonia's bid to join the European Union and NATO is blocked by a long-running dispute with Greece over the country's name.