Britain on Wednesday offered the strongest signal to date that a bomb was to blame for the Saturday crash of a Russian airliner over Egypt’s Sinai Peninsula by halting flights to and from the resort city of Sharm el-Sheikh amid concerns about airport security.

In a Wednesday night television appearance, British Foreign Secretary Philip Hammond cited “a significant possibility that the crash was caused by an explosive device on board the aircraft.” He said that flights from Britain to Sharm el-Sheikh would be suspended indefinitely and that the thousands of Britons already in the city would return home under “emergency procedures for additional screening.”

As the British government suspended flights, a U.S. official said that intelligence potentially indicates that the Russian plane was brought down by a bomb. But the official cautioned that the information was still being vetted. The official, who spoke on the condition of anonymity to discuss an ongoing investigation, would not describe the kind of intelligence that was being examined.

The Islamic State’s affiliate in Egypt has repeatedly claimed responsibility for the crash, which left 224 people dead when a Metrojet airliner broke up midflight and scattered debris across seven square miles of desert.

The group reiterated its assertion Wednesday in an audio clip that appeared to taunt Russian and Egyptian officials who have sought to play down suggestions that terrorism was to blame.

Here's what we know about the deadly Russian plane crash. (Monica Akhtar/The Washington Post)

“Search the wreckage of the plane and bring forth your black box and analyze it. Show us your expertise, and prove that we did not cause the plane to crash,” the group said. “We shall reveal in the coming days the mechanics of bringing down the plane, at the time we want and through the method we deem best.”

Britain’s announcement will significantly heighten speculation that the group’s claims are accurate — particularly the theory that a bomb was smuggled on board, either in the cabin or the cargo hold. Such attacks have become rare in recent decades as airport security has intensified. If a bomb was to blame, the crash could expose previously unknown vulnerabilities.

Egyptian Foreign Minister Sameh Shoukry told CNN on Wednesday that the government had increased security at the country’s airports. But he denied that the precaution was an indication the plane had been brought down by terrorists.

Even with the heightened measures, Britain’s decision to suspend all flights suggested that the country’s intelligence services have deep apprehensions about the state of security at the Sharm el-Sheikh airport.

Britain first announced a pause in flights from Sharm el-Sheikh in the late afternoon and said a team of experts was on the ground evaluating security measures. But a spokesperson for 10 Downing Street said they were not satisfied with what they found, prompting Hammond to announce an indefinite suspension of flights and to warn Britons against all but essential travel to the Red Sea resort, which is a favorite of British tourists.

Hammond said the decision was made “very reluctantly,” acknowledging the likely impact on the Egyptian economy. But he said that “we have to put the safety and security of British nationals above all other considerations.”

An estimated 20,000 Britons are on the ground in and around Sharm el-Sheikh. The 10 Downing Street spokesperson, who spoke under customary rules of anonymity, said there would be no flights back to Britain on Thursday and that bringing British citizens home would “take time.”

Ireland’s aviation authority said Wednesday that it would follow Britain’s lead and advise its country’s carriers to avoid Sharm el-Sheikh.

White House press secretary Josh Earnest said that no U.S. carriers regularly operate out of the Sinai. “The airport in question . . . is not the last point of departure into the United States for any airline, including foreign airlines that do operate in the Sinai Peninsula,” he said.

The suspension of flights came just a day before British Prime Minister David Cameron is due to host Egyptian President Abdel Fatah al-Sissi in London. A statement from 10 Downing Street on Wednesday said that Cameron and Sissi had spoken by phone on Tuesday evening to discuss security at the Sharm el-Sheikh airport.

Britain has not been directly involved in the investigation into why the Russian plane crashed, and the Downing Street spokesperson would not comment on what information had triggered Wednesday’s suspension.

But Stephen Wright, an aviation expert at the University of Leeds, said the highly unusual move suggests officials are concerned that airport security has been compromised. They are likely focused, he said, on the procedures for vetting airport personnel — and the possibility that someone working at the airport helped smuggle a bomb on board the Metrojet plane.

“If it’s happened once, it can happen again,” he said. “It’s a very astute move to err on the side of caution.”

Earlier this year, police found and defused two improvised explosive devices near the arrivals hall at the international airport in Cairo. And security experts have warned that corruption inside the Interior Ministry and other government institutions can make the system vulnerable.

Security is generally tight in Sharm el-Sheikh, one of Egypt’s most important tourism destinations and the site of a number of major international conferences. Dozens of international and regional airlines fly to the city.

A senior official with Metrojet had said Monday that the company ruled out pilot error or a technical malfunction as the cause — seeming to point to malfeasance. The comments elicited a stern rebuke from Russian officials.

U.S. Director of National Intelligence James R. Clapper Jr. said Monday that there was no “direct evidence of terrorist involvement yet” but that it could not be ruled out.

Several carriers fly direct between London and Sharm el-Sheikh, including British Airways, Monarch Airlines and EasyJet. British passengers late Wednesday tweeted that they had been left stranded at the airport, with airlines providing no information about when their flights might depart or how they will be able to leave the city.

Air France, Lufthansa and Emirates had earlier announced they would avoid flying over the Sinai Peninsula while Saturday’s crash remains under investigation. British Airways and other U.K.-based carriers, however, had not joined them.

Egypt’s Civil Aviation Ministry said Wednesday that the plane’s cockpit voice recorder had been damaged in the crash, which could slow the investigation.

Investigators have taken particular note that the tail of the aircraft was found about three miles from much of the rest of the debris, which fluttered down in pieces from an explosion tens of thousands of feet above the Earth’s surface.

It the tail broke loose first, the pilot would lose the ability to control the plane. That circumstance would be reminiscent of the worst crash of a single airplane in history, the 1984 crash of Japan Airlines Flight 123 that killed 520 people.

Cunningham reported from Cairo. Heba Habib in Cairo, Ashley Halsey III and Adam Goldman in Washington, and Karla Adam in London contributed to this report.

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Today’s coverage from Post correspondents around the world