Investigators on Tuesday began examining the two black boxes from the Russian airliner that crashed in Egypt's Sinai Peninsula, as President Abdel Fattah al-Sisi said the probe would take time.

Sisi described a claim by the Egyptian branch of the Islamic State jihadist group that it downed the Airbus A321 that crashed on Saturday killing all 224 people on board as "propaganda."

Egyptian President Abdul Fattah al-Sisi has dismissed as "propaganda" claims by a branch of the Islamic State group that they were responsible downing a Russian plane, the BBC reported Tuesday.

His remarks came as a top U.S. intelligence official said it was "unlikely" IS had been involved in the Kogalymavia airline disaster over the Sinai peninsula that killed 224 people on Saturday.

The Russian passenger jet that crashed in Egypt killing all 224 people on board came down due to "external" factors, the airline said Monday, as relatives began identifying their loved ones in Saint Petersburg.

Senior Kogalymavia executive Alexander Smirnov said that "no technical failures" could account for why the Airbus 321 would have broken up in mid-air, as investigators have said happened.

A Russian passenger plane with 224 people on board crashed in a mountainous part of Egypt's Sinai Peninsula on Saturday, leaving "many" dead including 17 children, officials said.

The chartered plane had taken off from the south Sinai resort of Sharm el-Sheikh bound for Saint Petersburg and lost contact with air traffic control 23 minutes later before crashing in the restive peninsula.

Russian President Vladimir Putin is "winging" it in Syria, U.S. Director of National Intelligence James Clapper says, in damning remarks of the Kremlin strongman's actions in the war-torn country.

"I personally question whether he has some long-term strategy or whether he is being very opportunistic on a day-to-day basis," Clapper told CNN late Thursday. "And I think his intervention into Syria is another manifestation of that."

Key nations backing Syria's warring sides will seek to thrash out differences at talks Thursday as Iran readies to make its debut at international negotiations on ending the four-year conflict.

Russia, the United States, Saudi Arabia and Turkey, the four major powers meeting in Vienna, support opposite sides in a war the U.S.'s top diplomat has described as "hell".

The first Russian serviceman confirmed dead in Syria was laid to rest on Wednesday as his family and friends disputed the official explanation that his death was caused by suicide.

Authorities said 19-year-old soldier Vadim Kostenko hanged himself at Russia's airbase in the Syrian province of Latakia due to relationship trouble but his loved ones said they would never believe he had taken his own life.

Russia's military on Tuesday said that a soldier had killed himself while on duty in Syria, the first confirmed death of a serviceman in the war-torn country since Moscow launched its bombing campaign last month. 

The defense ministry in Moscow did not identify the soldier who it said committed suicide but said the serviceman appeared to have taken his life due to problems with his girlfriend.

Two men and a woman were convicted at trial in New York on Monday of illegally exporting high-tech electronics from Texas to Russian military and spy agencies.

They were among 11 people charged in the United States in 2012 over a scam that allegedly illegally exported millions of dollars worth of electronics to Russia under the guise of civilian deals.

Just a few weeks ago, economics student Firas Kiwani was planning to leave his home in the Syrian capital Damascus, convinced there was no future in his war-torn country.

But the launch last month of a Russian air campaign in support of the Syrian government has changed the 20-year-old's mind.