US Director of National Intelligence James Clapper said today he knew of no "direct evidence" that terrorism was to blame for the weekend plane crash of a Russian passenger plane in Egypt.
Speaking at a Washington defence summit, Mr Clapper said it was "unlikely" that the Islamic State group had the capacity to carry out such an attack, but added "I wouldn't rule it out."
"We don't have any direct evidence of a terrorist element yet," he said.
Earlier, the Russian airline whose jet crashed in Egypt killing all 224 people on board has said it came down because of "external" factors and could not have been caused by technical faults or human error.
The crash, in Egypt's Sinai Peninsula, could only have been the result of some other "technical or physical action" which caused it to break up in the air and then plummet to the ground, said Alexander Smirnov, deputy general director of the airline, Kogalymavia.
Mr Smirnov told a news conference in Moscow that "the only explanation, is some kind of external action", without giving further details.
Meanwhile, Russian Prime Minister Dmitry Medvedev called for a thorough investigation into the plane crash.
"The key task is to investigate in detail what caused the tragedy," Mr Medvedev said.
The Russian passenger plane had passed a safety review carried out by the Irish Aviation Authority earlier this year.
The Airbus aircraft was registered in Ireland through a leasing company but was operated by Russian airline Metrojet.
"The Irish Aviation Authority conducted an annual review of the aircraft certifications in support of its annual Certificate of Airworthiness renewal process and all certifications were satisfactory at that point in time," a spokesman said.
He added that oversight of the operation of the aircraft, including daily and monthly checks, was Russia's responsibility under International Civil Aviation Organisation rules.
Two inspectors from the Air Accident Investigation Unit and an expert from the Irish Aviation Authority left to travel to Egypt today to join the investigation into the cause of the crash.
The head of the Russia's Air Transport Agency said the airliner disintegrated at high altitude.
Meanwhile, a plane carrying the remains of 144 people killed in the crash landed in St Petersburg early this morning.
The remains were taken in a motorcade to a crematorium for identification, which will begin later today.
Family members have been providing DNA samples at a crisis centre set up close to the Pulkovo airport, now the site of an impromptu memorial where people are bringing flowers and cuddly toys to commemorate the victims, including young children.
Most of those who died were from St Petersburg and the surrounding region who were due home on Saturday from the popular Egyptian resort of Sharm el-Sheikh.
