The Security Service of Ukraine (SBU) says it had thwarted a plot to attack soccer's European Championship in France by arresting a Frenchman attempting to cross from Ukraine into the European Union via Poland armed to the teeth.

The SBU said in a statement Monday it had been following the 25-year-old since December, and allowed him to purchase Kalashnikov machine-guns, rocket propelled grenades, 125 kilograms of TNT and other weapons before he was arrested on the border between Ukraine and Poland last month.

The SBU didn't identify the man, but said he was driven by ultra-nationalist views and that he had spoken out against France's immigration policy, the spread of Islam and globalization.

The agency said he tried to establish contacts with Ukrainian troops "under the guise of volunteering" while in that country's war-torn eastern region. Ukrainian troops and Russia-backed separatists have been fighting in that region since April 2014, killing more than 9,400 people. It was not immediately clear which side of the conflict the French man supports.

"He also said that he wanted to perpetrate acts of terror in protest," the SBU said in a statement.

The suspect planned a series of attacks on bridges, railways, Jewish and Muslim places of worship, French government buildings including tax offices, and buildings connected to the 2016 European Championship, the SBU said.

It said the man tried to take the weapons across the border himself after failing to hire a smuggler.

A video released by the SBU purports to show the suspect loading weapons into a van which, moments later, is swarmed by security personnel. The man's face is blurred in the video and in other images released by the SBU.

Confiscated ammunition and explosives on a display at the Yahodyn border crossing. The SBU said the suspect purchased five machine-guns, two rocket propelled grenades and other weapons before his arrest. (SBU Press Service/Associated Press)

A French Foreign Ministry spokesman confirmed a Frenchman had been arrested in Ukraine and said the ministry was in contact with Ukrainian authorities.

French police initially reacted skeptically and said the detained man was not known to either police or intelligence services.

However, another police source close to the case said police had found a T-shirt with a badge carrying the insignia of an extreme right group at the suspect's home, as well as "several substances that could go into the mix of homemade explosives." The source did not elaborate.

'Need to improve'

Extremist attacks are a major concern for French authorities as they prepare to host the month-long tournament at stadiums in the Paris area and eight other cities, running from Friday to July 10. Islamic State in Iraq and Syria (ISIS) extremists have threatened France during the tournament, but authorities have not confirmed specific dangers.

France is deploying 90,000 security personnel for the tournament, and French President François Hollande said Sunday night that the threat of attacks won't stop Euro 2016 from being successful.

The deployment — which includes at least 10,000 military troops — is expected to be especially vigilant around transportation hubs and the Stade de France arena, which holds 80,000 people and was among the sites targeted during the deadly November terror attacks in Paris.

Paris police prefect Michel Cadot said Monday arrangements around Stade de France "need to improve."

Fans have complained that new precautions — including a two-metre high wall and triple security checks — created severe overcrowding during a match last month. Despite those measures, supporters still managed to smuggle flares into the venue and ignite fires in the stands.

A state of emergency is still in place across France following the November attacks, which killed 130 people.