LONDON (Reuters) - The British police said on Thursday they had charged a 26-year-old woman with being a member of the Islamic State in Iraq and Syria (ISIS) and inciting terrorism on social media, and in a separate case had arrested a 15-year-old girl on suspicion of a terrorism offence.

The woman, 26, from Burton-upon-Trent in central England, who cannot be named for legal reasons, was arrested by counter-terrorism officers at London's Heathrow Airport in February after arriving on a flight from Turkey.

The West Midlands Police said she had been charged with publishing messages on Twitter in October last year which encouraged people to commit or prepare acts of terrorism, and being a member of the banned organisation ISIS.

She is due to appear before magistrates in London later on Thursday.

Separately, London police said they had arrested a 15-year-old girl in east London on suspicion of preparing acts of terrorism. She is currently in custody awaiting questioning.

Some 700 Britons are estimated by the authorities to have travelled to Syria or Iraq, many to join ISIS, and on Wednesday police said they feared a family of 12 had been the latest to make the journey.

Britain has stepped up attempts to try to stop people going or arrest them on their return, while Prime Minister David Cameron has called on social media firms to do more to deal with online extremism, saying some young Britons had been radicalised "in their bedrooms".

"(ISIS') methods of murder may be barbaric, but its methods of recruitment, propaganda and communication use the latest technology," he told Parliament this week.