"A terrorist attack is among the number of possible reasons why this happened," Russian Prime Minister Medvedev said in an interview with "Rossiyskaya Gazeta" state newspaper on Monday.

Until this statement, Russia had refrained from blaming the crash on a possible act of terror, and declared the question of its cause as open. Egyptian officials, however, insist that there is no evidence yet of an attack on the aircraft which jihadists linked to "Islamic State" claim to have downed.

At the same time, an insider with the Egyptian investigation into what caused the downing of the airplane told Reuters news agency that they could say with "90 percent" certainty that a bomb had caused the tragedy.

"The indications and analysis so far of the sound on the black box indicate it was a bomb," said the investigator, who asked to remain anonymous.

So far, there are two main theories for the disaster. One of them speculates that a bomb placed on the Airbus A321 flying from Sharm el-Sheikh to St. Petersburg exploded, killing all 224 people on board, most of whom were Russian tourists.

Another theory claims that a part of the 18-year-old plane's fuselage was damaged more than a decade earlier, succumbed to the cabin pressurization and ripped apart.

British Foreign Secretary Phillip Hammond told reporters on Monday that it "now looks more likely than not to have been an explosive device smuggled on to the plane" by operatives or loyalists of the "Islamic State" group.

Returning home

Some 80,000 Russians vacationers were heading home from Egypt with special planes sent by Moscow, which suspended all flights to Egypt last week amid security concerns.

Tens of thousands of foreign tourists, including 20,000 Britons, were stranded in the resort after flights were canceled due to a growing suspicion that it was a bomb that caused the Metrojet to crash.

Moscow said that about 25,000 Russian tourists had so far returned home on more than 100 flights, while Prime Minister David Cameron's office said some 5,000 Britons were back in the UK. Medvedev said he does not expect flights to Egypt to resume any time soon, saying that "it will take time" to ensure safety of travelers in Egypt.

The north of the Sinai Peninsula has been the scene of nearly two years of an Islamist insurgency, which has left scores of people dead, including Egyptian army and police officers.

dr/kms (AP, AFP, dpa)