WASHINGTON (AFP) - US authorities plan to bolster security at some airports in Europe and the Middle East with direct flights to the United States, officials said on Wednesday.

Amid concern terror groups are developing new explosives to circumvent airport security, US Homeland Security Secretary Jeh Johnson announced unspecified steps that would be carried out in "the coming days," without saying which airports would be affected.

"We are sharing recent and relevant information with our foreign allies and are consulting the aviation industry," Mr Johnson said in a statement.

After an assessment of security threats, Mr Johnson said he had directed the Transportation Security Administration "to implement enhanced security measures in the coming days at certain overseas airports with direct flights to the United States."

Mr Johnson said that "we will continue to adjust security measures to promote aviation security without unnecessary disruptions to the traveling public."

The airports were located in the Middle East and Europe, according to an official at the Department of Homeland Security, who spoke on condition of anonymity.

"Information about specific enhancements is sensitive as we do not wish to divulge information about specific layers of security to those who would do harm," a second DHS official, who asked not to be named, told AFP.