LONDON — British Prime Minister David Cameron declared Thursday it was “more likely than not” that a bomb brought down a Metrojet flight packed with Russian vacationers — a scenario that Russian and Egyptian officials dismissed as premature speculation.

Cameron said he had grounded all British flights to and from Egypt’s Sinai Peninsula because of “intelligence and information” indicating that a bomb was the likely culprit in the crash Saturday that killed all 224 people onboard. The move stranded thousands of British tourists at the Red Sea resort of Sharm el-Sheikh.

Cameron said he had “every sympathy” with the Egyptians, who rely heavily on tourism, but added he had to “put the safety of British people first.”

The radical Islamic State group has claimed that it brought down the plane in the Sinai, a report rejected by Russian and Egyptian officials as not credible. Egypt is fighting an Islamic insurgency in the area where the plane crashed.

“We don’t know for certain that it was a terrorist bomb … (but it’s a) strong possibility,” Cameron said at his London office at 10 Downing St. shortly before a previously scheduled meeting with Egypt’s president. “There’s still an investigation taking place in Egypt. We need to see the results of that investigation.”

Later, Cameron and Russian President Vladimir Putin spoke about the crash in a phone call. The Kremlin said Putin told the British leader it’s necessary to rely on data yielded by the official crash probe while assessing the reason for the crash. It also said the two men discussed the joint fight against terror.

A British team was working with Egypt to tighten security at the Sharm el-Sheikh airport so flights could resume. Cameron said “we want to start as soon as possible” to bring tourists home, and empty planes will be flying out from Britain to do that, but it would take some time.

Egypt will have to put in tighter, long-term security measures before British flights will resume flying on a regular basis, British Transport Secretary Patrick McLoughlin told the House of Commons on Thursday.

He said short-term measures, including different luggage-handing arrangements, would allow the estimated 20,000 British citizens in the Sharm el-Sheikh area, many of them tourists, to fly home.

Putin’s spokesman, Dmitry Peskov, insisted that investigators were working on all possible theories as to why the Airbus A321-200 crashed Saturday in Egypt’s Sinai Peninsula just 23 minutes after taking off. He said naming just one possibility was mere speculation.

“One cannot rule out a single theory, but at this point there are no reasons to voice just one theory as reliable — only investigators can do that,” Peskov told reporters in Moscow.

Russian Foreign Ministry spokeswoman Maria Zakharova said if Britain did have information about a bomb on the plane, it’s “really shocking” that hasn’t been shared with Russia.

Egypt, meanwhile, condemned the British travel ban as an overreaction, with its minister of civil aviation, Hossam Kamal, insisting Thursday that the country’s airports meet international security standards.

“The investigation team does not have yet any evidence or data confirming this hypothesis” of a bomb bringing down the plane, he said.