Macedonian police say they have arrested seven people on suspicion that they fought alongside Islamist insurgents during the wars in Syria and Iraq.
The suspects, all Macedonian nationals aged between 23 and 41, were arrested in an overnight operation on August 7 on international warrants, the Interior Ministry said.
They will be charged for "participation in a foreign army," it said.
The Associated Press (AP) news agency reported that the suspects were among some 600 foreign fighters captured on the battlefield in Syria by U.S.-backed Syrian Democratic Forces as they liberated Syrian territory last year that was once held by the Islamic State (IS) extremist group.
AP said the United States has been pressuring allies to bring the captured fighters home to face justice.
Under a Macedonian antiterrorism law, it is illegal to participate in any paramilitary group or armed conflicts abroad. If convicted, the men will face jail sentences of up to 10 years.
Most of the ethnic Albanians who make up one third of Macedonia's 2 million population are Muslim, but practice a moderate form of Islam.
Macedonian officials have estimated that 130 Macedonians left the country and joined the Islamic State, with about 20 of those killed in fighting in Syria and Iraq.
IS at one point in 2015 held nearly a third of the territory in Iraq and Syria, including major cities such as Mosul and Raqqa, which it proclaimed to be the capital of its so-called "caliphate."
But last year, campaigns by U.S.-backed forces and the Russian-backed Syrian Army pushed IS out of most of the territory it held, and hundreds of IS fighters and their family members were taken into captivity.
Western officials have expressed concern that those returning from Syria and Iraq could represent a security threat in the Balkans, which was riven by ethnic and sectarian wars during the 1990s.