The Airbus 321-200 went down in northern Egypt only 23 minutes after taking off on Saturday from Sharm al-Sheikh, a popular Egyptian Red Sea resort.

The Airbus, carrying mostly Russian citizens, was returning to Saint Petersburg in Russia from Sharm al-Sheikh, Egyptian authorities said Saturday.

Russian President Vladimir Putin has declared Sunday a day of mourning while the authorities have set up an emergency center at the Saint Petersburg airport, where friends and relatives gathered, awaiting news of the victims.

The Egyptian investigators found one of the plane's two flight recorders or "black boxes," at the scene. A senior air traffic controller said the plane's pilot told him in his last transmission that he was having trouble with the radio system.

The pilot also reportedly said he wanted to make an emergency landing before losing contact with ground control. The initial information suggests that the plane crashed due to a technical fault, security sources in North Sinai said.

Authorities in both Russia and Egypt have started investigations into the incident. Moscow is to "insist" that the flight recorders are processed in Russia, a source within the Russian investigation team told the Interfax news agency.

The 18-year old Airbus went through a detailed check last year, according to the TH&C Holding company which owns the Kogalymavia airlines which operated the flight.

German Chancellor Angela Merkel, US Secretary of State John Kerry, British Prime Minister David Cameron, as well as representatives of several other countries, have offered their condolences to Russia.

IS claims

"You who kill will be killed," a militant group affiliated to "Islamic State" (IS) in Sinai, said in a Twitter message on Saturday.

IS, which is active in Egypt's Sinai Province, said it brought down the passenger plane "in response to Russian airstrikes that killed hundreds of Muslims on Syrian land."

Russian authorities told Interfax news agency the claim "can't be considered accurate."

"Based on our contacts with the Egyptian side, the information that the airplane was shot down must not be considered reliable," Russian transport minister Maxim Sokolov told media.

In January 2014, Sinai-based militants shot down a military helicopter, but they are now known to have the capability to strike a high-flying passenger plane. The Russian airliner that crashed on Saturday was cruising at 31,000 feet when it lost contact with air traffic controllers, according to Egyptian aviation officials.

Egyptian security forces are battling against IS and its affiliated militant networks in Sinai, much of which has been declared a military zone.

German carrier Lufthansa and Air France-KLM said they would not fly over Sinai until the investigations about the crash were complete.

Shs/jm (AFP, AP, dpa, Reuters, Interfax)