PARIS (AFP) - French rail unions held make-or-break talks with employers Monday to try to end a six-day strike that threatens to disrupt the Euro 2016 football championships already clouded by security fears.

Tensions were raised on Monday as Ukraine's intelligence service said it had arrested a Frenchman who was allegedly planning to stage 15 attacks before and during the tournament.

France is still struggling with the impact of the rail strike, which combined with heavy flooding in Paris and other parts of the country to leave train services severely affected on Monday. Unions say the offer on the table from state-run rail operator SNCF is "not up to the mark".

The government is trying to reach agreements with unions in various disputes that have dogged France for weeks, before the host nation kicks off Euro 2016 against Romania in Paris on Friday.

Around 1.5 million visitors are expected for the one-month championship. President Francois Hollande warned rail workers and pilots from Air France, who are threatening to strike this month, would receive little sympathy if they spoiled the experience of spectators attending one of the world's biggest sporting events.

"No-one would understand if the trains or the planes... prevented the smooth functioning and affected the transport of spectators," Hollande said. "I want this to be resolved," he said.

SNCF said the rail strike was costing it more than 20 million euros (S$31 million) a day. France has been gripped by a wave of strikes, protests and blockades over controversial labour reforms pushed through parliament by the Socialist government.

The tournament is also taking place with France on high alert following jihadist attacks in Paris in January and November last year that left nearly 150 people dead.

Hollande tried to rally the subdued nation on Sunday, acknowledging that the threat of an attack during the tournament could not be discounted but that people "must not be daunted".

"This threat will last for a long time, unfortunately," he told France Inter radio."So we must do everything to ensure that Euro 2016 is a success."

The French authorities have mustered 90,000 security personnel to guard stadiums and "fan zones" where spectators will gather to watch the matches on giant screens.

Air France pilots, in a long-running internal conflict within the airline, are threatening to start a four-day strike on June 11, when the tournament is in full swing, and a major protest is planned in Paris on June 14.

Several oil refineries remain out of operation despite a vote by unions to resume work after stoppages had created petrol stations that left motorists queuing at filling stations last month.