London's Metropolitan Police arrested four men on Tuesday on suspicion of commissioning or preparing acts of terrorism, just two weeks after police in the United Kingdom arrested nine others on similar charges.

London's police force searched several homes and vehicles in connection with the arrests, according to a report from CNN.

Police didn't release specifics about the arrests of the suspected plot but did say that it was part of "an ongoing investigation into Islamist-related terrorism." The four arrested on Tuesday were between the ages of 20 and 21.

"They have all been taken to police stations in central London and remain in custody," the Metropolitan Police said in a statement.

British security advisers have said that there was an increased possibility of terrorist activity aimed at the U.K. after they sent fighter jets to conduct airstrikes against the Islamic State in Iraq, the Guardian reported.

Since a U.S.-led coalition, which includes the U.K., has been attacking the Islamic State, the Joint Terrorism Analysis Center has raised the terrorism threat level to "severe," its second-highest level.

On Sept. 25, British police arrested nine men, including Islamic cleric Anjem Choudary, in London on suspicion of being members of organizations banned in the U.K. and supporting terrorism.

During that round of arrests, Scotland Yard raided 18 properties in London and one property in Stoke-on-Trent in what it said was an ongoing investigation against terrorism. Those arrested last month were between the ages of 22 and 51.

Formerly the leader of Islamist group al-Muhajiroun, which was banned in 2010 by the U.K. government, Choudary is suspected of radicalizing hundreds of fighters for the Islamic State.

The identities of those arrested Tuesday have not been released, so it's unclear if and how they are connected to the Islamic State.