European countries, including France and Italy, are stepping up security in the wake of the recent terrorist attack at a concert hall in Moscow that killed 139 people.

It came as Russian President Vladimir Putin acknowledged on Monday that Friday's attack was carried out by extremists, but suggested it was also to the benefit of Ukraine and that Kyiv may have played a role.

Putin made the remarks during a Kremlin meeting as France joined the United States in saying intelligence indicated Islamic State was responsible for the killings.

In France, the government upgraded the terror threat level to its maximum on Sunday, with the country a frequent target of the Islamic State terror group, which has claimed responsibility for Friday's bloodshed.

The highest alert of the "Vigipirate" system means that security forces will maintain a more visible presence on French streets and be posted in front of possible targets such as government buildings, transport infrastructure or schools.

President Emmanuel Macron said on Monday the ISIS entity believed to be behind the killings — known as Khorasan, or ISIS-K, an offshoot of ISIS in Afghanistan and Pakistan — had also sought to attack France.

French Interior Minister Gerald Darmanin said the Paris Olympics, which begins on July 26, were an obvious future target.

French organizers have faced persistent questioning over their choice of the opening ceremony, which is set to take place outside of the athletics stadium for the first time.

Athletes are instead set to sail down the Seine River in a flotilla of riverboats in a made-for-TV extravaganza that has been resisted by some security officials because of the challenges for police.

French Prime Minister Gabriel Attal said that 4,000 extra soldiers would be deployed nationwide in the days to come.

Threat real

"The terrorist threat is real, it's strong," he told reporters, adding that two plots had been thwarted already this year.

Similar measures were also announced in Italy. While the security threat level had already been raised to maximum immediately after the attack, the latest measures are focused on boosting safety over Easter.

At least 10.5 million Italians are expected to travel during the Easter holidays, according to a survey by Italy's hotel and tourism association Federalberghi.

Many foreign tourists also visit Italy during this period, with the number estimated at 3.3 million during Easter 2023.

In Denmark, the security alert was raised last week due to an increased terrorism threat. The threat level will now remain at maximum.

In the United Kingdom, the current national threat level stands at "substantial" or midlevel, the third highest in the country's five-tier system.

Gunmen stormed Moscow's Crocus City Hall on Friday evening before setting the building on fire. Hundreds of Russians have laid flowers outside the hall to remember the victims.

Alexander Bastrykin, head of Russia's Investigative Committee, told the Kremlin meeting the death toll had risen to 139, with 182 people wounded.

Four men of Tajik origin were remanded in custody on terrorism charges at a Moscow court. Three others, also of Tajik origin, were remanded in custody on suspicion of complicity.

"We know that the crime was carried out by the hand of radical Islamists with an ideology that the Muslim world has fought for centuries," Putin said in remarks posted on the Telegram messaging app.

He did not directly mention the IS group and said the assailants had been trying to flee to Ukraine, saying there were "many questions" to be examined.

"The question that arises is who benefits from this?" Putin said. "We know by whose hand the crime against Russia and its people was committed. But what is of interest to us is who ordered it."

Agencies and Xinhua contributed to this story.