Elaine Duke, the acting secretary in charge of defending the US from terror attacks, warned small scale knife and vehicle attacks were only a means to gain funding from supporters and keep followers engaged in their unholy war.
It comes as MI5 boss Andrew Parker warns the UK is facing an unprecedented threat from terrorists.
Speaking at the US embassy in London, Ms Duke said: “The terrorist organisations, be it ISIS or others, want to have the big explosion like they did on 9/11. 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.”
Her comments follow a wave of terror in the UK, including the London Bridge, Westminster Bridge and Manchester arena attacks which had a combined death toll of 40 while leaving 347 injured.
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.”
The security expert, who was elevated to the position after General John Kelly became President Trump’s right-hand man, claimed the UK faces unique challenges on terror compared to the US.
She added: “We think the level of terrorist threat against the United States too is extremely high.
“I think that it is challenging for you because you have the proximities to other countries, the ease of movement from some of the terrorist safe havens is a little easier for you, but we feel the terrorist threat is very high in the United States.”
On Tuesday the head of MI5 warned Britain is facing an unprecedented terror threat, with security services at risk of being overwhelmed by “more terrorist activity coming at us, more quickly”.
Andrew Parker said he had never seen counter-terrorism operations working at such speeds in his 34 years at the UK intelligence agency.
He added that 20 planned attacks on British soil had been foiled in the past four years, seven of them in the past seven months.
All of them had been masterminded by Islamic extremists, he said.
The prolific nature of the plots meant it was becoming “harder to detect” every attack being planned.
MI5 staff were also “deeply affected” whenever an attack is carried out, Mr Parker told an audience in London.
He added: ”They get up and come to work every single day to make terrorist attacks less likely and to keep the country safe.
“They are constantly making tough professional judgements based on fragments of intelligence; pinpricks of light against a dark and shifting canvas.”