American officials have ordered some overseas airports with direct flights to the US to intensify screening of electronic devices.

Transport officials said in a statement passengers could be asked to switch on devices, and equipment that does not power up would not be allowed on board.

An official told the BBC that London’s Heathrow was among the airports.

The US announced new security measures last week, apparently in response to a terror threat, but gave no details.

Analysts say the changes appear to be in response to intelligence that Islamic militants in Syria and Yemen are developing bombs that could evade airport security.

American officials said earlier that there was a “credible” threat, but did not link the security changes to any specific intelligence.