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“Emergency measures” were being drafted Thursday to rescue thousands of stranded vacationers in the Red Sea resort of Sharm el-Sheikh as investigators focused on an ISIS-linked bomb as the likely cause of the Metrojet plane crash.

Intelligence reports indicate that a device placed on the Russian charter jet caused it to break up in midair Saturday, killing all 224 on board, U.S. officials told NBC News. Investigators are said to be focused on the possibility that ISIS operatives or sympathizers were directly involved.

The emerging probability of a deadly security lapse triggered Britain to suspend flights and advise against all but essential air travel to the Egyptian tourism hub, while at least three top officials at the airport, including its chief of security, were fired.

Related: No Evidence So Far That Jet Was Bombed, Egypt Official Says

U.K. Foreign Secretary Philip Hammond said Thursday there is a “significant possibility that the Russian aircraft was brought down by an explosive device on board.”

He said it was safe for tourists to remain in the resort for now, but added: “We will not allow any U.K.-bound aircraft to take off from Sharm el-Sheikh until we are absolutely certain that it is safe for it to do so.”

Britain was “looking with the airlines and the Egyptians at introducing emergency measures that will allow us to bring back the British people … safely and securely,” he added.

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Sarah Cotterill said she was about to "board the plane" to London Wednesday when the flight suspension was announced. She told BBC that "after spending about three hours at the airport we've been bussed back to our hotel, and that's where we are at the moment."

Ireland also suspended flights, but Russia — which recently began a military intervention in Syria against ISIS — continues to allow flights to Sharm el-Sheikh.

A senior Russian lawmaker said Britain's decision to stop flights from Egypt's Sharm el-Sheikh resort was motivated by London's opposition to Russian action in Syria, the RIA Novosti news agency reported.

"There is geopolitical opposition to the actions of Russia in Syria," said Konstantin Kosachev, a senior member of Russia's upper house of parliament.

A spokesman for Russian president Vladimir Putin told reporters that Moscow "cannot rule out a single theory" about the crash.

However, British PM David Cameron said intelligence reports indicated it was “more likely than not” that a bomb was responsible.

“We cannot be certain that Russian airliner was brought down by a bomb but because it is a possibility … it is right to act,” he said.

I returned from Sharm El sheikh airport Friday morning just b4 plane crash. Security seemed shoddy to me and officials overstretched. — Paul Hudson (@Hudsonweather) November 4, 2015

U.S. officials also stressed that it's too early to conclude that for certain that ISIS bombed the jet, telling NBC News that mechanical failure remains a possibility.

Metrojet, whose official company name is Kogalymavia, briefly grounded its three other Airbus A321 aircraft for safety checks Thursday but all had been passed as fit to fly, RIA news agency reported citing cited Russian state transport agency, Rostransnadzor.

Debris from crashed Russian jet lies in Sinai, Egypt. STR / EPA

Security experts have warned that fallout from Saturday’s crash could dramatically raise the stakes for the U.S.-led coalition in its battle against ISIS.

Among the likely scenarios if it turns out that ISIS has developed the capability to target planes is that Russia would sharply increase its military presence in Syria. A senior U.S. military official in the Middle East told NBC News that Russia would likely retaliate "heavily and militarily” is a bomb is found to be the cause.

Thursday’s developments came as the first of the crash victims was laid to rest at a funeral service in a medieval church in city in northern Russia.

Nina Lushchenko, 60, who worked in a school cafeteria all her life, was honored in a a whitewashed 16th-century church in Veliky Novgorod, about 100 miles south of St. Petersburg.

Family and friends remembered her as a good mother and grandmother who had considered taking her granddaughter with her on vacation but later decided against it.