Two teenagers, including a girl, have been arrested on suspicion of terrorism offences as part of an ongoing investigation linked to Islamist extremism, Scotland Yard said here on Sunday.

The arrest of an 18-year-old man in Essex, near London, was made on suspicion of encouraging terrorism contrary to Section 1 of the UK Terrorism Act 2006 (TACT) and a 17-year-old girl was arrested at an address in east London as part of the same investigation.

It follows the arrest of a 13-year-old boy earlier this week on suspicion of dissemination of terrorist material contrary to Section 2 of TACT 2006.

The investigation related to alleged offences linked to extreme Islamist ideology, the Metropolitan Police said.

Those arrested remain in custody while enquiries continue following the court granting the Met Police warrant of further detention.

We have seen three teenagers arrested this week by Met officers for terrorism offences, albeit as part of separate investigations, said Commander Richard Smith, head of the Met Police's Counter Terrorism Command.

We need to let this investigation run its course, but it is a further indication of a concerning upward trend in police action against younger people for terrorism-related matters. Across the country, police are working hard to try and prevent young people from being radicalised, and acting quickly when it is suspected that offences have been committed, he said.

Police rely on information from the public in our mission to tackle terrorism. If you see or hear something unusual or suspicious and think someone may be engaging in terrorist activity, trust your instincts and contact police, he said.

The 18-year-old man was first arrested on Wednesday under the Police and Criminal Evidence Act (PACE) before being detained under TACT on Thursday.

A warrant of further detention application was granted by Westminster Magistrates' Court, allowing detectives to keep him in custody until May 25, the Met Police said.

The 17-year-old girl was arrested under Section 41 of the Terrorism Act 2000 and remains in custody for questioning until May 27.

The Met Police Counter Terrorism Command has called on anyone who thinks a friend or relative is becoming radicalised or drawn into a path towards terrorism to contact the force as part of the ACT Early strategy or Action Counters Terrorism Early.

There is also an app called iREPORTit for people to report such terror-related concerns.