One of the four people arrested by police in London on Tuesday has links to the Islamic State group in Syria, The Guardian reported Wednesday, citing counterterrorism officials. The four men had reportedly been under surveillance for some time by the MI5, UK’s domestic counterintelligence and security agency.

The four men, aged 20 and 21, were arrested from west and central London on Tuesday while planning a “significant” attack in the country, according to media reports. Security officials told BBC that the arrests “may have foiled the early stages” of the planned attack.

“It is difficult to talk in a great deal of detail about it, but it is a quite serious case and it's one of a series of arrests that we've had over the last few weeks, which taken together for me confirm that the drumbeat around terrorism has changed,” Metropolitan Police Commissioner Bernard Hogan-Howe told BBC. “A lot of it is linked back to Syria and Iraq.”

Scotland Yard reportedly said that the men were currently being held at police stations in central London “on suspicion of being concerned in the commission, preparation or instigation of acts of terrorism.” According to media reports, the arrests were reportedly carried out after investigators had gathered enough evidence of an imminent attack.

“These arrests and searches are part of an ongoing investigation into Islamist related terrorism,” a Scotland Yard spokesperson reportedly said. However, the nature of the alleged plot has not yet been revealed.

The arrests come just a week after a militant of the Islamic State group, believed to a British national named Omar Hussein, urged jihadists in a video titled “Message of the Mujahideen,” to carry out attacks on the UK for supporting the U.S.-led airstrikes in Iraq.

The UK has maintained its threat level at severe -- its second highest level -- since August, after reports that terrorist groups were planning to carry out an attack against the U.S. and its allies.