A threat has reportedly been made against planes flying from Heathrow to the US.

An alert from non-governmental counter-terrorism organisation SITE Intelligence Group on Friday warned of a threat being made by a "pro-Islamic State Twitter account".

It reportedly threatened attacks on planes "flying from Heathrow to the US" across the July 4 US Independence Day holiday weekend.

The alert also warned "there will be a device placed in either Heathrow, LAX or JFK airports".

Transport minister Lord Ahmad of Wimbledon said "all steps necessary to keep the public safe" are being taken.

Heathrow Airport declined to comment on the apparent threat and referred inquiries to the Department for Transport.