LONDON -- The Islamic State of Iraq and Syria (ISIS) on Sunday issued a claim of responsibility for an attack that killed seven people in a busy area of Central London.

In a statement more than 24 hours after the attack, the group's Aamaq news agency quoted a "security source," saying, "A Unit of Islamic State Fighters carried out London attacks yesterday."

ISIS has often made such claims not just when it has sent attackers, but when extremists carrying out deadly plots were inspired by the group's ideology.

Prime Minister Theresa May on Sunday called for tougher measures to contain extremism in Britain.

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May, facing an election Thursday, said the recent attacks, including an earlier one outside Parliament in March, are not directly connected, but that "terrorism breeds terrorism" and attackers copy one another. She also said five credible plots have been disrupted since March.

"They are bound together by the single evil ideology of Islamist extremism that preaches hatred, sows division, and promotes sectarianism," she said.

The events unfolded Saturday night around 10 p.m. GMT when a van mowed down pedestrians along the London Bridge. Three men fled the van with large knives and attacked people at bars and restaurants in nearby Borough Market, police and witnesses said. The attack unfolded quickly, and police said officers had shot and killed the three attackers within eight minutes.

It marks the third attack this year that ISIS has claimed in Britain, after the bombing in Manchester and a similar attack in the heart of London in March.

The three attackers in Saturday's attack have not been identified.