Shooting broke out for a second day on Sunday in northern Macedonia after clashes in the restive Balkan nation left at least eight officers and 14 gunmen dead, police said.

"Eight police officers were killed and 37 were injured" in shooting in the city of Kumanovo that began Saturday at dawn, a police spokesman said, adding that 14 bodies in uniforms were also found, allegedly those of members of a "terrorist group".

As the crisis-hit country declared two days of mourning, the European Union warned of the danger of escalation in a part of the nation hit by an ethnic Albanian insurgency in 2001.

The clashes have been blamed by the government on fighters from across the border. One in four Macedonians is an ethnic Albanian.

Hospital sources in Skopje earlier confirmed that a sixth police officer had died from wounds suffered in battles with the gunmen.

"A policeman has died while another one is in critical condition," a hospital source told AFP on condition of anonymity.

Police patrol in the streets of Kumanovo, northern Macedonia, after clashes with unidentified gunmen near the Kosovo border left six officers dead on May 9, 2015 AFP Photo

The government previously said that five police officers had been killed and some 30 injured in the violence in Kumanovo, some 40 kilometres north of Skopje.

"There are victims among the terrorists but for the time being we cannot give the exact number," Interior Minister Gordana Jankuloska told reporters late Saturday.

Sporadic fire from automatic weapons could be heard overnight and on Sunday as helicopters patrolled overhead, the state-run MIA news agency reported, citing witnesses.

"The situation on the ground is still very risky," Jankuloska said.

EU warns against escalation  

"Any further escalation must be avoided," said the EU's enlargement commissioner Johannes Hahn.

Policemen watch a carrier truck transporting an armored vehicle as Macedonian special forces engage in a police action against an alleged group of armed Albanians, in Kumanovo, the Former Yugoslav Republic of Macedonia, 10 May 2015.. Photo : AFP

The violence broke out when police moved in to dismantle what officials called a "terrorist group" that allegedly entered Macedonia illegally from a neighbouring country.

Jankuloska said advancing security forces were met with "violent resistance" from snipers, grenades and automatic weapons.

Jankuloska said the gunmen were planning a "terrorist attack" on state institutions, and had accomplices in Kumanovo.

Local media suggested the assailants came from neighbouring Kosovo, populated mostly by ethnic Albanians, with some reports indicating five members of the group had been killed.

Media broadcast images of armoured police vehicles deployed across Kumanovo, with officers clad in bullet-proof jackets.

On Saturday dozens of people, mostly women, children and the elderly, fled the besieged zone, some of them being evacuated by police, according to an AFP photographer.

A drone flies over Kumanovo, Macedonia May 10, 2015. Photo AFP

Police noted that "difficult terrain" was hampering the operation.

Calls for calm, dialogue  

The incident comes less then three weeks after around 40 ethnic Albanians from neighbouring Kosovo briefly seized control of a police station on Macedonia's northern border, demanding the creation of an Albanian state in Macedonia.

Ethnic Albanians make up around one quarter of Macedonia's 2.1 million people.

Both Albania and Kosovo, as well as Macedonia's ethnic Albanian junior ruling party DUI, strongly condemned Saturday's clashes and called for calm.

In Tirana, the foreign ministry issued a statement calling for restraint "to prevent further deterioration of the situation which is not in favour of Macedonia's democratic stability and prosperity."

Pristina echoed that by imploring "all sides to find a solution through a political dialogue."

A Police officer arrests an ethnic Albanian man on the street as Macedonian special forces engage in a police action against an alleged unidentified armed group, in Kumanovo, the Former Yugoslav Republic of Macedonia, 09 May 2015. Photo AFP

Serbia sent additional forces to its border with both Kosovo and Macedonia, state-run Tanjug news agency reported.

The 2001 Macedonian conflict ended with an agreement providing more rights to the community, but ties between ethnic Macedonians and Albanians remain strained.

The violence in Kumanovo erupted amid political tension in Macedonia, where the government and centre-left opposition continue have been trading accusations including claims of wiretapping and million-euro bribes.

The crisis has not only undermined Macedonia's already weak institutions, but also sparked concerns within the 28-nation European Union that Skopje hopes to join.

The US embassy in Skopje this weekend issued a statement expressing condolences "to the families of the people killed and to those injured."

An elderly woman is evacuated in an armored vehicle near a police checkpoint in Kumanovo, Macedonia May 10, 2015. Photo AFP