(Adds PM Cameron comment, detail)
LONDON, July 3 (Reuters) - Britain has stepped up security at airports after U.S. officials said they were concerned that al Qaeda operatives in Syria and Yemen were developing bombs that could be smuggled onto planes.
"This is something that we've discussed with the Americans and what we've done is put in place some extra precautions and extra checks," Prime Minister David Cameron told reporters.
"The safety of the travelling public must come first. We mustn't take any risks with that. I hope this won't lead to unnecessary delays but it's very important that we always put safety first and we do," Cameron said.
The United States requested tougher security at overseas airports with nonstop flights to its cities.
The request came as U.S. security sources said bombmakers from the Nusra Front, al Qaeda's affiliate in Syria, and Yemen-based Al Qaeda in the Arabian Peninsula were believed to be working to develop explosives that could avoid detection by current airport screening systems.
The main concern is that militant groups could try to blow up U.S.- or Europe-bound planes by concealing bombs on foreign fighters carrying Western passports who spent time with Islamist rebel factions in the region, the U.S. sources said.
A spokeswoman for Britain's transport ministry declined to give any further details on the security measures but a witness at London's Heathrow Airport boarding a flight to the United States said shoes, bags and electrical equipment such as laptops were being checked.
Heathrow is the world's third busiest airport and the busiest in Europe, serving 191,200 passengers per day. American Airlines, Delta and United Airlines fly from the airport.
Britain, which lost 52 people when suicide bombers struck the London transport system on July 7, 2005, said its current threat level was 'substantial', a level that means there is a strong possibility of an attack.
That level has been in place since July 2011 when the level was lowered to 'substantial' from 'severe', a level that means an attack is highly likely. (Reporting By Costas Pitas, editing by Guy Faulconbridge)