Moscow suspended passenger flights to Egypt, and the United States imposed new air travel security requirements in the wake of the crash of a Russian jet in Egypt, as Western officials pointed to the conclusion it was brought down by a bomb.
A group affiliated with Islamic State has claimed responsibility for the crash of an Airbus A321 operated by a Russian carrier on Saturday (local time) that was bringing holidaymakers home from a resort on Egypt's Sinai Peninsula.
All 224 people on board were killed in what the militants described as revenge for Russian air strikes in Syria that began more than a month ago.
While no official investigation has confirmed that claim of responsibility, countries have been cancelling flights and announcing new precautions, leaving tens of thousands of European and Russian tourists stranded at Red Sea resorts.
The US Department of Homeland Security announced new security measures on Friday, including tighter screening of items before they are brought on board aircraft, for flights to the United States from some foreign airports in the region.
US President Barack Obama and British Prime Minister David Cameron have already said the crash might have been caused by a bomb.
Moscow initially rebuked Western countries for drawing such conclusions too quickly, but President Vladimir Putin's decision to suspend Russian flights suggests the Kremlin is no longer trying to avert attention from that theory.
The American TV network NBC cited unidentified US officials as saying communications between Islamic State leaders in Raqqa, Syria, and people in the Sinai Peninsula included boasts about the downing of a Russian passenger jet over the area.
"They were clearly celebrating," NBC Nightly News quoted a US official as saying.
The "chatter" included a boast of how the plane was brought down.
Separately, a new video released by Islamic State purports to show Islamic State leaders in Aleppo congratulating their counterparts in Sinai after the crash, CNN reported.
French TV station France 2 said on its website that the sound of an explosion could be heard on the black boxes recovered from the plane, according to an investigator who had access to them. The investigator ruled out engine failure, it added.
British and US spies intercepted "chatter" from suspected militants as well as internal communication about the incident from one other government that suggested a bomb, possibly hidden in luggage in the hold, had downed the airliner, Western intelligence sources said.
The intelligence sources, who spoke on customary condition of anonymity, said the evidence was not categorical and there was still no hard forensic or scientific evidence to support the bomb theory.
"We still cannot be categorical, but there is a distinct and credible possibility that there was a bomb," one source said.
A Sinai-based group affiliated with Islamic State, the militant group that has seized swathes of Iraq and Syria, has claimed responsibility for the crash, which if confirmed would make it the first attack on civil aviation by the world's most violent jihadist organisation.
But Moscow, which launched air strikes against Islamist fighters including Islamic State in Syria more than a month ago, has said it is premature to reach conclusions that the flight was attacked.
Egypt, which depends on tourism as a crucial source of revenue, has said there was no evidence that a bomb was to blame.