Isis supporters threaten attacks on Heathrow and US airports over Fourth of July weekend

American security officials said the threat was not believed to be credible

A minister said security was high at Heathrow and other UK airports (Simon Calder)

A minister has said security services will take “all steps necessary” to keep the public safe after Isis supporters threatened to attack Heathrow Airport over the weekend.

A Twitter account threatened planes flying from the UK’s largest airport to the US over the coming days, when Americans are celebrating Independence Day.

“There will be a device placed in either Heathrow, LAX or JFK airports,” the message claimed, according to monitors at the Site Intelligence Group.

JFK airport was among the airports threatened (Shutterstock)

John F Kennedy International Airport (JFK) in New York and Los Angeles International Airport (LAX) are among the busiest in the US.

Lord Ahmad of Wimbledon, a transport minister, said security remains high with the UK’s national threat level currently set at “severe”.

“We all need to be vigilant to the global threat of terrorism – in the UK we keep all aspects of aviation security under constant review and work closely with our international partners to mitigate risks,” he added.

“We will continue to take all steps necessary to keep the public safe, but for security reasons we do not comment in detail on specific measures or operational matters.”

Heathrow Airport declined to comment on the apparent threat, which had no confirmed link to the so-called Islamic State or its strongholds in Syria and Iraq.

Officials in the US said they had no information to suggest any plot was underway and did not consider the claim to be an “executable plan”, while the FBI previously said it knew of no specific or credible threat over the Fourth of July weekend.

Brian Levin, from the Centre for the Study of Hate and Extremism at California State University, told the LA Times that such threats are fairly common as part of a propaganda effort to spread fear.

“This kind of chatter isn’t unusual around 4 July and other big holidays,” he said. “You have to assess the ability to carry out the plan.”

Isis has issued numerous videos and propaganda statements threatening the UK and other “Crusader” nations in the US-led coalition bombing its strongholds.

A video was posted online earlier this year showing two British supporters threatening Heathrow, Gatwick Airport and Downing Street following the Brussels attacks.

At least 22 people were killed by Isis militants in a shooting attack on a restaurant in Bangladesh on Friday, while Tuesday’s bombings at Istanbul airport were also blamed on the group.

It has called for increased attacks during the Muslim holy month of Ramadan and analysts predicted that the group would attempt more terror plots abroad as it continues to suffer heavy losses in Syria, Iraq and Libya.