The Euro 2016 football tournament is a potential target for Islamic State (IS), the head of the EU's law enforcement agency has said.
However, Europol director Rob Wainwright told the Press Association the risk is "not necessarily high" given the massive security operation set to be mounted by the French authorities.
On Tuesday the US government issued a travel alert to Americans. warning of the risk of potential attacks throughout Europe.
The State Department singled out the tournament, which will be held in France from June 10 to July 10.
Mr Wainwright said: "I have no doubt that the Euros are on a potential target list for IS, for obvious reasons. That's a pretty obvious assumption. The threat is high, I think.
"The risk isn't necessarily high because, to counter-balance that threat, I see a huge amount of security preparations being taken by the French authorities, with extra police and military drafted in.
"We are dealing with a country a bit like Britain where they know how to do their security and they've learned a lot of lessons also from the attacks last November."
Terrorists "would want to attack" the football tournament, he said, but "it would be difficult for them to pull it off".
However, he added that it is "difficult to get the threat down to zero".
Euro 2016 will take place amid an atmosphere of high alert following atrocities in Paris and Brussels.
Last November, 130 people were killed when terrorists launched co-ordinated attacks on the French capital, targeting bars, restaurants and the national stadium, the Stade de France. Then in March, 32 people died when Brussels was hit by suicide bombings at the airport and on the Metro.
French authorities have extended a state of emergency until the end of the tournament, as well as the Tour de France cycling race, which will be held from July 2 to July 24.
Some 2.5 million football fans are expected in stadiums, including 1.5 million foreign visitors.
Last week UK police said up to 500,000 British fans, around half of whom do not have tickets, are expected to travel to France for the competition.
The US State Department travel alert, which expires on August 31, said: "Euro Cup stadiums, fan zones, and unaffiliated entertainment venues broadcasting the tournaments in France and across Europe represent potential targets for terrorists, as do other large-scale sporting events and public gathering places throughout Europe."
It set out steps for American citizens to take, including exercising vigilance in public places, as well as advising them to be prepared for additional security screening.
The alert also cites the Catholic Church's World Youth Day event in Krakow, Poland, which is expected to attract up to 2.5 million visitors.