BERLIN (REUTERS) - Germany has tightened security at its airports after US authorities said they were concerned that Al-Qaeda operatives in Syria and Yemen were developing bombs that could be smuggled onto planes.

Germany's interior ministry said in a statement on Tuesday that authorities had introduced enhanced security measures following a request from the United States.

"At German airports, the authorities have already implemented higher levels of security checks for passengers and baggage," said the ministry.

It said passengers flying from Germany directly to the United States would have to remove technical equipment, such as laptops, from protective covers and turn it on to make sure it worked.

The United States said last week it planned to step up security at overseas airports with direct flights to its cities. That includes airports in Europe, Africa and the Middle East.

Britain took precautionary measures similar to Germany's last week.

The Nusra Front, Al-Qaeda's affiliate in Syria, and Yemen-based Al-Qaeda in the Arabian Peninsula, or AQAP, are believed to be working together to try to develop explosives that could avoid detection by airport screening systems, US national security sources said.

The main concern is that militant groups could try to blow up US-or Europe-bound planes by concealing bombs on foreign fighters carrying Western passports who spent time with Islamist rebel factions in the region, the sources said.