LONDON » British Prime Minister David Cameron declared today 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 responsibility for bringing down the plane in the Sinai, a claim rejected by Russian and Egyptian officials as not credible. Egypt is, however, fighting an Islamic insurgency in the area where the plane crashed and Russia is supporting the Syrian government with airstrikes against IS targets.

"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. The two men discussed the joint fight against terror.

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

After meeting with Cameron, Egyptian President Abdel-Fattah el-Sissi said British officials had sent a security team to check Sharm el-Sheikh airport 10 months ago and were satisfied with the results.

"They checked the security actions, they were happy with that," he told a news conference, speaking through an interpreter. Egypt stands "completely ready to cooperate with all of our friends" to ensure the safety of foreign tourists.

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 today.

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 from Sharm el-Sheikh. 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 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.

Germany’s Lufthansa Group, however, announced it, too, would cancel flights of its subsidiary airlines — Edelweiss and Eurowings — to Sharm el-Sheikh. Flights to Cairo would not be affected, the company said.

In Sharm el-Sheikh, British tourists backed the flight ban but also spoke of their fondness for the resort.

"We understand why the government have done it, but I am really worried for the Egyptian people because — particularly in the Red Sea resorts — they are so dependent on tourism," said Paul Modley, a 49-year-old Londoner who has travelled to the town seven times in the last nine years.

Metrojet suspended all flights of Airbus A321 jets in its fleet after the crash. The company has ruled out a pilot error or a technical fault as a possible cause of the crash, drawing criticism from Russian officials for speaking with such certainty too soon.

A Metrojet flight bound for Sharm el-Sheikh was delayed today after the plane damaged its forward landing gear. Officials said the flight would be conducted by another Russian airline.

Philip Baum, editor of Aviation Security International, said a bomb could be placed on a plane in an endless number of ways, including by someone with airside access or by those on catering teams or maintenance crews.

"The options are almost too many to consider," he said in a telephone interview from Gambia.

U.S. satellite imagery detected heat around the jet just before it went down Saturday, two U.S. officials told The Associated Press. The infrared activity could mean many things, including a bomb blast or an engine on the plane exploding due to a malfunction.

Another U.S. official briefed on the Metrojet crash told the AP on Wednesday that intercepted communications played a role in the tentative conclusion that the Islamic State group’s Sinai affiliate had planted an explosive device on the flight. The official added that intelligence analysts don’t believe the operation was ordered by Islamic State leaders in Raqqa, Syria, but possibly planned and executed by its affiliate in the Sinai.

All the U.S. officials spoke on condition of anonymity because they weren’t authorized to discuss intelligence matters publicly.

Russia’s top aviation official, Alexander Neradko, said investigators are pursuing several theories as to why the plane crashed, including looking for traces of explosives on victims’ bodies, their baggage and on the plane debris as well as studying other "aspects linked to a possible terrorist attack."

He said the crash probe is likely to take several months and called for caution in speculation about the likely causes.

At the Sinai crash site, forensic evidence, including the airplane’s black box, was still being analyzed. Rescue teams have retrieved 140 bodies and more than 100 body parts combing a 15.4-square mile area and hoped to finish their search for remains and wreckage by this evening.

Egyptian Antiquities Minister Mamdouh Eldamaty rejected the U.S. and British bomb allegations outright.

"It is not a terror act. It was an accident," he declared in Luxor as authorities opened three tombs to the public for the first time in an effort to encourage tourism.

In the ancient Russian city of Veliky Novgorod, 100 miles south of St. Petersburg, family and friends said a tearful goodbye today to Nina Lushchenko, the first Metrojet crash victim to be buried.

The 60-year-old school cafeteria worker was remembered as a good mother and grandmother after a church service in a whitewashed 16th-century church overlooking the Volkhov River.

Vasilyeva reported from Moscow. Brian Rohan from Luxor, Egypt, Ken Dilanian in Washington, Jill Lawless and Danica Kirka in London, Vladimir Isachenkov in Moscow and Dmitry Lovetsky in Veliky Novgorod, Russia, also contributed.