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 officials from Russia and Egypt tried to dismiss as premature speculation.

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

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.

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

“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.”

He said he would call Russian President Vladimir Putin later Thursday to discuss the crash.

A British team is working to tighten security at the Sharm el-Sheikh airport with an eye toward resuming flights. Cameron said “we want to start as soon as possible” to bring tourists home, and empty planes will be flying out from Britain to bring people back.

But the British leader cautioned it would take “some time” to repatriate the thousands of Britons in the resort of Sharm el-Sheikh.

A spokesman for Putin, Dmitry Peskov, insisted that aviation investigators were working on all possible theories as to why the Airbus A321-200 carrying 224 people crashed Saturday in Egypt’s Sinai Peninsula, killing everyone on board. 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 had information about the bomb, it’s “really shocking” that it hasn’t shared it with Russia. She urged Britain to immediately give any such information to the investigators.

Egyptian officials have condemned Britain’s travel ban as an overreaction. Egyptian President Abdel-Fattah el-Sissi, who was in London on a visit Thursday, has called the IS claim “propaganda.”

Russia’s top aviation official, Alexander Neradko, said in televised remarks Thursday that investigators are pursuing several theories as to why the plane crashed. He said they are looking for traces of explosives on the victims’ bodies, their baggage and the plane debris as well as studying other “aspects linked to a possible terrorist attack onboard.”

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

In the ancient city of Luxor on Thursday, Egyptian Antiquities Minister Mamdouh Eldamaty rejected the U.S. and British allegations outright.

“(The crash) is not a terror act. It was an accident,” he declared as authorities opened three tombs to the public for the first time in an effort to encourage tourism. “(It’s) very sad what happened, but we have to wait for the result of the investigation.”

Egypt’s minister of civil aviation, Hossam Kamal, insisted Thursday that the country’s airports comply with international security standards.

He said, regarding U.S. and British allegations that the Russian flight may have been downed by a bomb, “the investigation team does not have yet any evidence or data confirming this hypothesis.”

Metrojet suspended all flights of Airbus A321 jets in its fleet after the crash, the Russian Federal Transport Agency said Thursday. 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.

Intercepted communications played a role in the tentative conclusion that the Islamic State group’s Sinai affiliate planted an explosive device on the plane, said a U.S. official briefed on the matter. He spoke on condition of anonymity because he wasn’t authorized to discuss intelligence matters publicly.

The official and others said there had been no formal judgment rendered by the CIA or other intelligence agencies, and that forensic evidence from the blast site, including the airplane’s black box, was still being analyzed.

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 the Islamic State’s affiliate in the Sinai, which operates autonomously.

In Sharm el-Sheikh, British tourists said they understand their government’s move to suspend flights. Paul Modley, a 49-year-old Londoner, has travelled to Sharm el-Sheikh seven times in the last nine years.

“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 Modley.

On the ground in the Sinai, rescue teams have retrieved 140 bodies from the scene and more than 100 body parts. Russian rescue workers, combing a 40 square kilometer (15.4 sq. mile) area, should be finishing their search for remains and wreckage by Thursday evening, according to Emergency Situations Minister Vladimir Puchkov.

Grief continued to roil St. Petersburg and its suburbs, as mourners brought more flowers, candles and paper planes to the city’s imperial-era square and the airport where the crashed Metrojet flight had been due to land.

In the ancient Russian city of Veliky Novgorod, 160 kilometers (100 miles) south of St. Petersburg, the first crash victim was buried Thursday after a church service in a whitewashed 16th-century church overlooking the Volkhov River.

Family and friends said goodbye to Nina Lushchenko, 60, who worked in a school canteen, remembering her as a good mother and grandmother.

(AP)