London: A British schoolgirl who fled to Syria to join the Islamic State (IS) terror group has reportedly married an Australian-born jihadist who has threatened to carry out attacks in Britain.

Amira Abase, 16, left the UK in February with two friends and was tracked down earlier this month to the IS-held territory of Raqqa.

There, she has reportedly married Abdullah Elmir, one of the youngest Western fighters to have joined IS, a 'Sunday Telegraph' report said.

Dubbed "the Ginger Jihadi" due to his ginger hair colour, Elmir fled Sydney last year for Syria and has since appeared in several IS propaganda videos.

The news came as Prime Minister David Cameron gets set for a major IS-related anti-terror meeting on Tuesday.

He has invited acting Opposition Labour leader Harriet Harman to the high-level security meeting about the threat posed by IS extremists.

The UK government is seeking support from Labour MPs to extend the RAF's air campaign to strikes against IS targets in Syria, as well as Iraq.

Labour has hinted it would not oppose action in Syria, as it did in 2013.

Harman will join senior ministers and military chiefs at the National Security Council meeting.

The last time a Labour leader received such an invitation was in 2013.

A 10, Downing Street spokesman said, "The PM thought it was important to ensure the leader of the opposition was fully briefed on the current situation".

The National Security Council meeting comes as ministers are facing criticism for advising UK holidaymakers to return home from Tunisia amid fears of another terror attack.

Last month, 30 Britons were killed when a gunman opened fire on tourists at a popular Tunisian holiday resort. The IS claimed responsibility for the attack.

Tunisia has accused the UK of playing into the terrorists' hands and some tourists complained they were being ordered out of the country.