But Russian and Egyptian authorities pushed back Thursday on suggestions that a bomb brought down Metrojet Flight 9268 over Egypt's Sinai Peninsula, saying there's no evidence yet to support that theory.
The debate over what caused Saturday's crash, which killed all 224 people on board
, comes as officials say their investigation is far from finished.
How? One U.S. official told CNN that "specificity" in chatter surrounding the crash of the Russian jet drew the attention of the U.S. intelligence community. The official says "the specific nature of the discussion" that officials monitored made them take notice.
The intelligence also suggests someone at the Sharm el-Sheikh airport helped get a bomb onto the plane, another U.S. official said.
But the intelligence isn't definitive, Obama said in an interview Thursday
with CBS News affiliate KIRO in Seattle.
"We're going to spend a lot of time just making sure our own investigators and own intelligence community find out what's going on before we make any definitive pronouncements," he said. "But it's certainly possible that there was a bomb on board."
Cameron said Thursday that he couldn't confirm "with certainty" why the Russian commercial jet crashed. Still, he said, the possibility was enough of a reason to keep British citizens from flying back for several days this week from Sharm el-Sheikh, a popular tourist destination in Egypt, until safety measures at the resort's airport could be bolstered.
Egypt is leading the crash investigation. Russia, France, Germany and Ireland also have investigators on the ground. But the United States and the United Kingdom aren't part of the investigative team combing over forensic evidence from the scene.
Egyptian Civil Aviation Minister Hossam Kamel said investigators have found no evidence to support the theory that a bomb caused the plane to crash.
Neither the United States nor the United Kingdom has shared intelligence about a possible bomb with Egyptian authorities, Egyptian officials said.
"I don't reject any conclusion. But I would only be able to assess that conclusion if information was to be shared," Egyptian Foreign Minister Sameh Shoukry told CNN's "The Situation Room." "I believe that this information has a direct bearing on both the investigation and our status, (with) this incident having happened on our territory. And I would have expected that if there is information, that it would have been shared with those immediately concerned."
In a phone conversation with Cameron on Thursday, Russian President Vladimir Putin said data from the official investigation should form the basis for assessments of what caused the crash, the Kremlin said in a statement
First funeral in Russia
Almost all of the victims on the doomed plane were Russian, and the first funeral for one of them took place Thursday.
The service was for one of the five residents of the city of Veliky Novgorod who have been identified, the city's mayor office told CNN on Thursday.
A memorial service was held in St. Boris and Gleb Church for crash victim Nina Luschenko, the office said. Among the 224 crash victims, 15 of them were from the Novgorod region, including two children.
Novgorod local and regional authorities will provide financial assistance to the victims' families, at more than $2,300 per victim, the mayor's office said.
Why some suspect ISIS involvement
The signs pointing to ISIS, another U.S. official said, are partially based on monitoring of the terrorist group's internal messages. Those messages are separate from public ISIS claims of responsibility, the official said.
In an audio message from ISIS' Sinai branch that was posted on terror-related social media accounts Wednesday, the organization adamantly insisted that it brought down the flight.
Typically, ISIS is quick to trumpet how and who carried out any attacks for purposes of praise and propaganda. To some, the fact that ISIS hasn't provided details in this case raises doubt about the group's repeated claims of responsibility.
Officials in Egypt and Russia
have said there's no evidence to support ISIS' claims.
Foreign tourists stranded in Egypt
Concerns that the plane might have been bombed left thousands of foreign tourists stuck in Egypt
About 1 million British tourists visit Sharm el-Sheikh every year, Cameron said Thursday at a news conference alongside Egyptian President Abdel Fattah el-Sisi
in London. Roughly 3,500 British nationals were due to fly out of Sharm el-Sheikh on Thursday. The most important thing, the British leader said, is for the tourists to come home safely.
Sisi said Egypt had already been cooperating with British teams on airport security going back 10 months.
After the meeting, Cameron's office said the two countries "agreed on a package of additional security measures that is being put into place rapidly" and that flights back to the United Kingdom will resume Friday.
Under the additional measures, passengers will be allowed to have only carry-on luggage on the flight. Checked baggage will be transported separately in a move CNN aviation correspondent Richard Quest called "extraordinary."
"Outbound flights from the UK to Sharm el-Sheikh remain suspended and the Foreign Office continues to advise against all but essential travel by air to or from Sharm el-Sheikh airport but we are continuing to work with the Egyptians to get back to normal service as soon as possible," Cameron's office said.
Sharm el-Sheikh, where Flight 9268 began its journey, is a beach resort at the southern tip of the Sinai Peninsula. The plane crashed about 300 kilometers (185 miles) north of the area, Egyptian authorities said.
Sinai has been a battleground between ISIS
-affiliated militants and Egyptian security forces in recent years. Hundreds have died in the fighting.
If a bomb did cause the Sinai plane crash, one conceivable motive for ISIS to attack a Russian airliner is that Russia started launching airstrikes in Syria
in September, saying it was coordinating with the Syrian regime to combat ISIS and other terrorists