There is a “huge risk of terrorist attacks in the European Union” over the coming holiday season, a senior EU official has said.
Her remarks came days after a tourist was fatally stabbed in Paris.
The EU was making an additional €30m (£26m) available for additional security, Ms Johansson said.
She did not specify whether there was any specific information which had led to the warning.
“We saw it recently in Paris, unfortunately we have seen it earlier as well,” she added ahead of a meeting of EU interior ministers.
A similar warning came from German Interior Minister Nancy Faeser who told reporters the EU had to keep a close eye on threats and propaganda, as there was a high “risk of further emotionalisation and radicalisation of violent Islamist perpetrators”.
Many European countries have seen a surge in hate crimes since Hamas gunmen attacked Israeli communities, leaving about 1,200 people dead and many others held hostage in Gaza. Israel’s invasion of Gaza has since left more than 15,000 people dead, according to the Hamas-run health ministry.
EU ministers were meeting after a deadly attack close to the Eiffel Tower on Saturday in which a 23-year-old German tourist called Collin B was stabbed to death, and his girlfriend and a British tourist were wounded.
The young German couple had visited Disneyland Paris, the Louvre and taken selfies in front of the Eiffel Tower before the attack.
Police said the 26-year-old French suspect, identified as Armand R, who comes from a non-religious Iranian family had pledged allegiance to the Islamic State group.
The man, who was arrested at the scene, had previously been imprisoned for planning a terror plot in the La Défense business district outside Paris.
In 2020, he was interviewed by French police for having communicated with Abdoullakh Anzorov, who murdered teacher Samuel Paty.
Anti-terror prosecutor Jean-François Ricard said that Armand R’s mother had raised concerns about his behaviour although there was no evidence at the time to take further measures against him.
Germany has also been on high alert for possible attack.
Last week two boys were detained in different parts of the country on suspicion of planning a militant Islamist attack on a Christmas market in the city of Leverkusen.
And a 20-year-old Iraqi who arrived in Germany last year is in custody on suspicion of planning a knife attack on a Christmas market in Hannover.
The head of German domestic intelligence in the eastern state of Thuringia, Stephan Kramer, has warned of the “considerable potential for danger” posed by Hamas-sympathisers, not just to Christmas markets but to major sporting events such as the Paris Olympics and the Euro 2024 football championships next year.