British police on Thursday charged a 26-year-old woman with inciting terror through Twitter and membership of the Islamic State group, and arrested a 15-year-old girl on suspicion of preparing an attack.

Police were also continuing an investigation into the disappearance of a family of 12, including three generations, believed to have travelled to Syria.

The 26-year-old woman was detained at Heathrow Airport after returning from Turkey.

The charges say that between September 30 and October 20 last year she published tweets intending people to be "directly encouraged or otherwise induced" to commit, prepare or instigate acts of terrorism.

Experts have said that Western women joining Islamic State militants are driven by the same ideological passion as many male recruits and should be seen potentially as dangerous cheerleaders for the cause.

A study by the London-based Institute for Strategic Dialogue earlier this year said that an estimated 550 Western women have travelled to Iraq and Syria.

- Missing family -

The Foreign Office meanwhile said it was in contact with Turkish authorities about the fate of the Bangladeshi-British family, including a grandfather aged 75 and his one-year-old grandchild.

"We are working closely with the Bedfordshire Police and Turkish authorities," the ministry said in a statement, referring to the police force responsible for Luton near London where the family lived.

Mohammed Akhtar Hussain, a family member who did not travel with them, told Channel 4 News that one of the 12, his sister, had initially been prevented from flying out of Britain by police officers.

A Bedfordshire police spokesman told AFP: "We have got an investigation that is ongoing, to make sure that they're OK and they're safe and that they want to be there."

The BBC cited sources as saying that the family was already in Syria.

In Westminster, Defence Secretary Michael Fallon raised the prospect of a parliamentary vote for Britain to extend its air strike campaign against the Islamic State group in Iraq to Syria.

Prime Minister David Cameron lost a vote on possible British military action in 2013 but the opposition Labour Party has recently signalled that it may reconsider its previous opposition.

"This is of course though a new parliament and it is for all members to consider how best to tackle ISIL, an evil caliphate that doesn't respect state boundaries," Fallon said, referring to the Islamic State group.

But Crispin Blunt, chairman of parliament's foreign affairs committee, said there was "no military necessity" for British involvement in Syria.

The latest developments come in the wake of an Islamist gun massacre at a Tunisian beach resort last Friday in which 38 tourists including 30 Britons were killed.

The first bodies of eight of the British victims were flown back on Wednesday and nine more were repatriated on Thursday.

It was the worst loss of British life in a terror attack since the 2005 suicide bombings in London in which 52 people were killed.