PARIS • Despite putting unprecedented security measures in place for Euro 2016, France remains deeply concerned over militants' ability to strike a soft target.

Millions of foreign visitors and the world's press are set to descend on the country for a month of sporting action from June 10 to July 10 - creating endless nightmares for its overstretched security services.

"From the point of view of preparation, we have done as much as possible. Everyone has been mobilised: police, paramilitaries, soldiers," a senior counter-terrorism official said. But he added: "To be totally honest, I'm worried."

And on Sunday, President Francois Hollande told France Inter radio: "This threat will last for a long time, unfortunately, so we must do everything to ensure that the Euro 2016 is a success."

The French government introduced a state of emergency in the wake of last year's attacks in Paris by the Islamic State in Iraq and Syria, allowing police to raid homes and place people under house arrest with minimal oversight.

But the challenge of monitoring those who have returned from Syria and Iraq, or who have sneaked into Europe using false passports or with the influx of refugees, has overwhelmed the security services.

"What really worries us above all is the guys who are already in Europe - guys that are already in Germany for example, that we haven't seen arrive, that the Germans haven't found, who have stayed quiet and waited," said the official.

An incident thousands of kilometres away has further darkened the mood: On May 13, in a town north of Baghdad, militants attacked a cafe where Real Madrid supporters regularly meet, killing 16 people with automatic weapons and grenades.

"It's like a postcard sent to Euro 2016 - a direct message," said the counter-terrorism official.

And tensions were raised further on Monday, when Ukraine's security services said they had arrested a suspected far-right extremist Frenchman with an arsenal of weapons and explosives who was allegedly planning "15 terrorist strikes" before and during the tournament.

Yesterday, Britain warned its citizens bound for Euro 2016 that stadiums, fan zones and transport hubs "represent potential targets for terrorist attacks", following a similar warning by the US last week.

AGENCE FRANCE-PRESSE