Greece's National Intelligence Service said Tuesday that it was at "a heightened state of vigilance" for suspected militants, keeping close tabs on radical Muslims, and had detected at least six foreign fighters with the terrorist group Islamic State transiting through the country in recent months.
The surveillance operation comes amid concern that the militant group, formerly known as Islamic State of Iraq and Syria, or ISIS, will retaliate for increased U.S airstrikes in Iraq and possible strikes in Syria.
"The threat level originating from Greece is very low because there are no verified indications of either dormant or active ISIS cells or splinter groups" within the country, a senior intelligence official said. "We are, however, at a heightened state of vigilance now, exchanging intelligence from the United States, Britain, France and others."
The official, who requested anonymity because of the sensitive nature of the topic, did not elaborate. Nor did he say whether Greece had issued international arrest warrants for Islamist militants in a bid to block the spillover of extremist violence from the Middle East.
Experts say that Greece, wedged between the West and the realm of Islam, could become a conduit for radicals traveling elsewhere in Europe.
In recent months, Greek intelligence has detected six Islamic State recruits traveling through the country, including a 23-year-old French national carrying a memory stick with instructions for making bombs.
The instructions carried by the Algerian-born Frenchman bore the slogan "in the name of God," according to Proto Thema, a leading Greek newspaper. His childhood friend, a Tunisian Frenchman who worked as a cook in upscale restaurants in Cannes before being recruited by Islamic State two years ago, was spotted by Greek authorities weeks later, reportedly returning from training in Syria.
"We couldn't arrest them because there were no legal and legitimate grounds to do so," the intelligence officer said. "We tipped off other agencies, instead, on their movements."
In another case, Greek authorities arrested a 43-year-old Frenchman of Russian descent for trying to drive a truck carrying weapons bound for Syria through a border crossing into Turkey.
Neither the intelligence officer nor other officials contacted Tuesday disclosed details of that arrest. They did not discount the possibility, however, of extraditing the man.
"Appropriate procedures are being followed," the intelligence officer said.
Alarm over Islamic State and its potential appeal to some Muslims emerged after a May attack on a Jewish museum in Brussels. Four people were killed in that attack; a Muslim Frenchman who had fought in Syria is being held.
Inundated by an influx of illegal migrants, cash-strapped Greece has been asking the European Union for more money and muscle to help push back tides of refugees -- and potentially Islamist militants -- sneaking in and out of the country.
Since the start of the year, Greece has deported more than 300 Syrians and Iraqis suspected of terrorism-related activities. What's more, with Greece neighboring Albania, the biggest source of Islamic State recruits in the Balkans, investigators warn that the passage of militants through this country could increase.
About 300 Albanian fighters have joined Islamic State and the militant group Al Nusra Front, which is also fighting in Syria, according to the International Center for the Study of Radicalization and Political Violence, a think tank based at London's King College. Forty of them were arrested and tried last month in Kosovo, among the poorest states in the Balkans.
"More than half of them were released because they had no idea what they were getting into," said Jeta Xharra, director of the Balkan Investigative Reporting Network in Pristina, the capital of Kosovo. "Many new recruits do not know either. They are just replying to Internet ads that are pouring in from Turkey, promising money in exchange for a year of adventure, like a gap year after college."
With unemployment and poverty gripping much of the Balkans, Xharra warned that destitute Muslims from Albania to the Turkish speaking areas of northern Greece could fall prey to the incitement of Islamist militants.
"Much of this has nothing to do with ideology," said Xharra. " Rather, money. All authorities have to do to track ISIS' foreign recruits is to follow the money trail."