People mourn as they lay flowers in memory of the plane crash victims at Dvortsovaya (Palace) Square in St. Petersburg. (AP Photo/Dmitry Lovetsky)

As investigators search for the cause of a plane crash that killed 224 people over the Sinai Peninsula on Saturday and Russia suspends all flights to Egypt, U.S. officials said they are taking unspecified precautionary measures to enhance the security of flights from the Middle East.

“While the facts and circumstances surrounding the tragic October 31 crash of Metrojet Flight 9268 on the Sinai Peninsula are still under investigation, Transportation Security Administrator (TSA) Peter Neffenger and I, out of an abundance of caution, have identified a series of interim, precautionary enhancements to aviation security with respect to commercial flights bound for the United States from certain foreign airports in the region,” Jeh C. Johnson, secretary for the Department of Homeland Security, said in a statement.

Johnson said the enhancements, also adopted by European aviation authorities, “are designed to provide an additional layer of security for the traveling public, and will be undertaken in consultation with relevant foreign governments and relevant passenger and cargo airlines.”

[Russia halts Eygpt flights amid widening probe into plane crash]

They include expanded screening and other “seen and unseen” security measures.

A DHS official, who asked not to be identified because he’s not authorized to expand on the secretary’s statement, said the new measures would be phased in as quickly as possible.

“This is all within TSA’s capabilities, within their power, within their authorities and resources,” the official said. “If we say ‘From now on a certain item poses a threat, no one’s allowed to bring that item aboard a plane.’ We can request that and if [other countries] refuse to [ban that item], we can refuse planes coming from that airport or from that country.”

[Russian airline spars with government over cause of plane crash]

He would not specify any category of items or cargo that might be banned from airplanes under the new order.

He said the new expanded screening and restriction of items brought aboard aircraft likely would apply primarily to those stowed in the cargo hold. In most airports worldwide, passengers and their carry-on bags are subject to x-ray machines, metal detectors and, in some cases, bomb-sniffing dogs.

[Obama: ‘There is a possibility’ bomb was aboard Russian jet]

“Obviously we could dial it all the way up and say you can’t bring anything at all in your carry-on, but that’s not workable, and it’s going to make everyone very angry,” the official said.

The official added that the tightened security measures were planned in conjunction with security agencies in Europe and the Middle East.

“It’s not like we’re deploying TSA agents abroad,” the official said. “There are TSA equivalents in every country, and these talks and consultations are always ongoing with them.”

The cause of the crash of the Russian jet, which could be the result of an explosive on board, a catastrophic mechanical failure, human error or an accidental explosion of fuel, remains a mystery.