Ukraine said on Monday it had arrested a suspected far-right French extremist with a huge arsenal of weapons who allegedly planned to attack the European Championships in France this week.
The arrest added to jitters over the staging of European football's showcase event in France after twin attacks in Paris last year claimed by the Islamic State group killed 147.
Preparations for the long-anticipated event have also been marred by rolling strikes that have paralysed transport and torrential rain that caused the River Seine to flood and force the temporary closure of the celebrated Louvre museum.
Ukraine's security service chief Vasyl Grytsak said the 25-year-old intended to blow up "a Muslim mosque, a Jewish synagogue, tax collection organisations, transportation checkpoints and numerous other locations".
Grytsak said he "expressed negative views about his government's approach to the immigration of foreigners into France, the spread of Islam and globalisation," suggesting the Frenchman was more of a far-right extremist or ultra-nationalist than a member of a jihadist group.
Video released by the SBU security service showed a man whose face was blurred out loading machineguns and other weapons into the back of a van.
Ukrainian authorities said his arsenal included 125 kilogrammes of TNT and 100 detonators as well as bullets and even grenade launchers.
The SBU said he was arrested on May 21 while trying to cross into Poland near the Ukrainian frontier town of Yagodyn.
Ukraine's secret service said it had become aware in December of the arrival of a French national who had begun "to establish contacts with a number of representatives in the (pro-Russian separatist) east".
90,000 security personnel 
The announcement came a day after French President Francois Hollande acknowledged that the threat of an attack during the month-long competition could not be discounted, but promised to "do everything to ensure that the Euro 2016 is a success".
Washington has already warned US citizens about the risk of attacks on stadiums as well as on fan zones where spectators will be gathering in large numbers.
The French authorities have mustered 90,000 security personnel to guard stadiums and fan zones.
The increasingly-anxious mood in France has cast a pall over a celebrated competition that unites the continent once every four years -- with matters not helped by a long-running labour dispute.
Make-or-break talks between unions and employers later Monday may decide whether the strikes will affect the influx of football fans from around the continent.
'Russian involvement' 
Grytsak said Ukrainian authorities suspected the French national was trying to obtain arms from unidentified groups in war-scarred eastern Ukraine.
"The Frenchman offered people with whom he was in contact (in the east) several thousand euros in order to have a Ukrainian citizen help him carry the load into Europe," he said.
"We therefore believe that the Russian security services may have set him up," he said.
Ukraine's eastern war zone has been awash with arms since an insurgency against the pro-Western government in Kiev erupted in April 2014.
The fighting has claimed the lives of nearly 9,400 people and sparked some resistance among EU member countries about signing a visa-free agreement with Ukraine with year.
Kiev and its Western allies blame Moscow for plotting and backing the 25-month war in reprisal for the February 2014 ouster of a Russian-backed president who had backed out of a landmark EU deal.
Ukraine has since signed that agreement and seen its relations with Russia effectively frozen since the revolt.