The United States said on Wednesday it would increase security at overseas airports with nonstop flights to the country and US officials cited concerns al Qaeda operatives in Syria and Yemen were developing bombs that could be smuggled onto planes.
The new security measures would be required at airports in Europe, Africa and the Middle East that have direct flights, the US officials said on condition of anonymity.
The Department of Homeland Security said "enhanced security measures" would be implemented in the next few days at "certain overseas airports with direct flights into the United States".
It did not specify which airports or what countries would be affected, nor did it say what triggered the extra precautions.
"We are sharing recent and relevant information with our foreign allies and are consulting the aviation industry," DHS Secretary Jeh Johnson said in a statement.
Johnson said he directed the Transportation Security Administration to implement the measures in the coming days. The move comes during the summer travel season and days before the July 4 holiday.
A US official said some of the new measures would involve additional inspections of passengers' shoes and property.
The official said Washington had legal authority to enforce new security requirements on foreign governments or airports because the flights go directly to the United States.
Earlier, law enforcement and security officials said the United States and European authorities were discussing measures that could include installation of additional bomb-detection machines.
Bombmakers from 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 current 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.
AQAP has a track record of plotting such attacks. It was behind a 2009 attempt by a militant with a bomb hidden in his underwear to blow up a Detroit-bound airliner.
There was no immediate indication US intelligence had detected a specific plot or time frame for carrying out an attack.
US officials believe Nusra and AQAP operatives have carried out operational testing of new bomb designs in Syria, where Nusra is one of the main Islamist groups fighting to overthrow President Bashar al-Assad, a national security source said.
The "stealth" explosives the bombmakers are trying to design include non-metallic bombs, ABC News in US reported.
But officials are especially worried that the recent battlefield successes of the Islamic State in Iraq and the Levant, or ISIL, an al Qaeda splinter group, have drawn a growing number of militants from America and Europe to the jihadist cause and they would have easy access to flights headed for American cities.