Isis and other Islamist terror groups are still plotting to bring down a plane in a similar attack to 9/11, Donald Trump's acting secretary of homeland security has warned.

Elaine Duke said jihadists were using lower level plots, such as knife or vehicle attacks, to raise money and keep their members engaged.

"The threat is still severe," Ms Duke told an event at the US Embassy in London, describing Isis as being in an "interim" period as it focuses on a bigger endgame.

Isis completely driven out of Raqqa by US-backed forces

"The terrorist organisations, be it Isis or others, want to have the big explosion like they did on 9/11," she said, the Daily Mail reported. "They want to take down aircraft, the intelligence is clear on that.

"However, in the interim they need to keep their finances flowing and they need to keep their visibility high and they need to keep their members engaged, so they are using small plots and they are happy to have small plots."

She added: "Creating terror is their goal and so a van attack, a bladed weapon attack, causes terror and continues to disrupt the world – but does not mean they’ve given up on a major aviation plot."

Syrian Democratic Forces take Raqqa from Isis – in pictures Show all 8 1 /8 Syrian Democratic Forces take Raqqa from Isis – in pictures Syrian Democratic Forces take Raqqa from Isis – in pictures Syrian Democratic Forces fighters celebrate victory in Raqqa atop of military vehicles REUTERS Syrian Democratic Forces take Raqqa from Isis – in pictures Fighters of Syrian Democratic Forces march past destroyed buildings as they celebrate victory REUTERS Syrian Democratic Forces take Raqqa from Isis – in pictures Fighters of Syrian Democratic Forces gesture the "V" sign in Raqqa REUTERS Syrian Democratic Forces take Raqqa from Isis – in pictures Fighters of Syrian Democratic Forces evacuate a civilian from the stadium REUTERS Syrian Democratic Forces take Raqqa from Isis – in pictures A civilian prays after she was rescued by fighters of Syrian Democratic Forces from the stadium REUTERS Syrian Democratic Forces take Raqqa from Isis – in pictures Syrian Democratic Forces fighters ride atop of military vehicles as they celebrate victory in Raqqa REUTERS Syrian Democratic Forces take Raqqa from Isis – in pictures Syrian Democratic Forces fighters celebrate victory in Raqqa REUTERS Syrian Democratic Forces take Raqqa from Isis – in pictures A fighter of Syrian Democratic Forces celebrates in Raqqa Reuters

Her comments came one day after the head of MI5 warned Britain was experiencing an unrelenting and unprecedented terror offensive.

Andrew Parker said in his 34 years working for the Security Service he had never faced such a relentless tempo of attempted and successful bombings, shootings and stabbings.

Ms Duke also said the US and Britain would push social media firms to do more to remove extremist propaganda at a meeting of G7 interior ministers.

Authorities say Isis propaganda has played a major part in radicalising people in the West but despite its defeat in its de facto capital Raqqa in Syria, Ms Duke said the group's online presence was likely to increase.

"I would surmise being able to put terrorist propaganda on the internet might become more imperative," she added.

She also warned that those who turned to violence by being radicalised by such material posed a bigger problem than the comparatively small number of fighters who had joined the militant group returning to US.