Prime Minister David Cameron
David Cameron has warned Isis militants cannot be appeased and Britain is at high risk of a terror attack AFP / Getty Images

A female British jihadist and supporter of Isis (known as the Islamic State) has condemned British Prime Minister David Cameron for "waging wars" on Muslims and tweeted that she wants his "head on a spike".

The woman, 18, is believed to have fled Britain to the IS stronghold of Raqqa in north-eastern Syria.

The girl's Twitter photo, posing in a niqab with a gun over her shoulder Twitter

"In sha Allah [God willing] a day will come when David Cameron's head will be on a spike as he continues to wage war on the awilya [protectors] of Allah... and strike terror in the hearts of the kuffar," she tweeted.

She added the punishment of stripping her of British citizenship would be "quite laughable".

"I really do not understand why Britain is threatening to remove our citizenship like we care lool its actually quiet laughable (sic)."

The female jihadist, using the Twitter handle @UmmKhattab_, posted photos of herself wearing a niqab to the social media website but her identity has not been revealed.

In other posts, she said she was "excited" at the possibility of US soldiers being "slayed" by IS fighters and said the black flag of IS should be raised over Downing Street.

She also told other foreign jihadists that moving to Syria was the "best decision".

"I might be only 18 but I know coming to shaam [is] the best decision. Staying in the UK completely diminishes your islam," she wrote.

The threats about Cameron come amid heightened fears that more and more female jihadists are flocking to Syria to fight for the terror group.

A group of female British jihadis are known to be heading a religious police force in Syria, entitled the al-Khanssaa Brigade, to prevent "un-Islamic" behaviour.

The teenager wrote that she hopes Cameron's "head will be on a spike". Twitter / @UmmKhattab_

Elsewhere, Britain has announced it is to give heavy machine guns and ammunition to the Iraqi government to assist their fight against IS, Britain's Ministry of Defence has confirmed.

Defence Secretary Michael Fallon confirmed the equipment was worth approximately £1.6m and an extra £475,000 in transport costs would be donated.

He said the UK was looking to promote "an inclusive, sovereign and democratic Iraq that can push back on ISIL [another term for Islamic State] advances and restore stability and security across the country; and working with the international community to tackle the broader threat that ISIL poses to the region and other countries around the world, including the UK".

IS launched its offensive on Iraq from its heartland of north-eastern Syria, capturing key Sunni towns and cities such as Mosul and Tikrit. It has now declared a "caliphate" that straddles the Iraqi-Syrian border and represents a greater landmass than that of the United Kingdom.

According to the UN, over 1.6 million people have been displaced by conflict in Iraq this year while 850,000 people fled their homes in August alone.