Ukraine and Russia traded accusations after US intelligence officials said a surface-to-air missile hit a Malaysia Airlines passenger plane carrying 298 people on Thursday.
US Vice President Joe Biden said the plane crash was "not an accident," adding that it was "blown out of the sky." Officials were divided over the origin of the missile that hit the plane, which crashed over eastern Ukraine.
In a CNBC interview, retired US Gen. Barry McCaffrey said, "This was a major strike, a deliberate strike to get an aircraft at that altitude."
The Ukrainian government, the pro-Russia separatists fighting in the region and Russian President Vladimir Putin denied responsibility for downing the aircraft. On his Facebook page, Ukraine's minister of internal affairs, Anton Gerashchenko, claimed that terrorists struck down the plane under Putin's orders.
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Putin said Ukraine bore responsibility for the downing of the plane, adding it would not have happened if Kiev had not resumed a military campaign against separatists.
Malaysia Airlines said Ukrainian aviation authorities told the company they had lost contact with Flight MH17, a Boeing 777 en route from Amsterdam to Kuala Lumpur, about 30 kilometers (20 miles) from Tamak waypoint, which is 50 kilometers (30 miles) from the Russia-Ukraine border.
"To bring down an airliner from 33,000 feet you need a good air defense weapon, not just a missile itself and also the radar. That says Russian, and that says to me either a separatist element or the Russians themselves," Retired Col. Ken Allard told CNBC.
Ukraine also has weaponry capable of hitting the passenger plane, Doug Richardson, editor of IHS Jane's Missiles & Rockets, said in a note.
But he noted, "The crew of the Command Post vehicle (containing the data display and control system for the launcher) are likely to have a good idea of the local air activity."
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Other types of launchers can be operated in "target-detection mode," automatically engaging targets in the radar's forward "field of view," he said.
Ukrainian President Petro Poroshenko said his country's armed forces did not take action against any airborne targets. "We do not exclude that this plane was shot down, and we stress that the armed forces of Ukraine did not take action against any airborne targets," he said.
Some noted that the strike may not have been deliberate.
"I just can't imagine the Ukrainian government, Russian government or the rebels wanting to shoot down another country's airliner with all those foreign nationals abroad. I think in all probability, it was accidental," Alan Diehl, a former National Transportation Safety Board investigator, told CNBC.
"This is going to be a real whodunit situation now that everybody is disavowing responsibility," Diehl said. "The good news is that US may have satellite assets around that border and we may be able to spot where, when and who fired that missile."
While the plane's route was over an area that has seen severe fighting between Ukrainian forces and pro-Russia separatist rebels in recent days, the carrier noted that the flight path had been declared safe and unrestricted by the International Civil Aviation Organisation. The International Air Transportation Association also noted the airspace wasn't subject to restrictions, MAS said.
A great deal of passenger airline traffic usually travels through the area on a daily basis, but many international carriers have now announced they will be changing their routes. Two other flights -- one from Singapore Airlines and one from Air India -- may have been within 30 kilometers of MH17 at the time it disappeared from radar, according to data from FlightRadar24, a website providing maps of real-time air traffic.
There were unconfirmed reports that the crashed plane's flight recorder (the so-called black box) had been located. According to a Dow Jones report, Ukrainian officials claim rebels have taken possession of the black boxes, complicating investigation efforts.
US President Barack Obama emphasized in a phone call with Poroshenko that all evidence from the crash site must remain in place until international investigators are able to examine all aspects of the tragedy. Meanwhile, UN Chief Ban Ki-Moon urged a "full and transparent international investigation" into the crash.
Pro-Russian separatists said they would have up to a three-day ceasefire in Eastern Ukraine to allow for recovery work at the site of the downed airliner, according to a RIA news agency.
Malaysia Airlines said there were at least 154 Dutch, 27 Australians, 43 Malaysians, including 15 crew and two infants, 12 Indonesians including one infant, nine Britons, four Germans, four Belgians, three Filipinos and one Canadian aboard the Boeing 777 jet.
There were a further 41 passengers whose nationality was not yet known. MAS said it would release the passenger manifest after notifying next of kin. The Australian newspaper reported that 108 of the passengers were to attend an AIDS conference in Melbourne, Australia.
Calling the downed plane a "terrible tragedy," Obama said the US will offer any help it can to determine what happened and why.
Malaysian Prime Minister Najib Razak said he was shocked by the reports and was also planning an investigation.
Boeing said it is aware of media reports and is gathering more information on the situation.
On Wednesday evening, a Ukrainian fighter jet was shot down by an air-to-air missile from a Russian plane, Ukrainian authorities said Thursday, adding to what Kiev says is mounting evidence that Moscow is directly supporting the separatist insurgents in eastern Ukraine. Security Council spokesman Andrei Lysenko said the pilot of the Sukhoi-25 jet hit by the air-to-air missile was forced to bail after his jet was hit.
Pro-Russia rebels, meanwhile, claimed responsibility for strikes Wednesday on two Ukrainian Sukhoi-25 jets. The Ukrainian Defense Ministry said the second jet was hit by a portable surface-to-air missile, but added the pilot was unscathed and managed to land his plane safely.
In the markets, Malaysian Airline System shares fell as much as 17.8 percent before retracing some losses to trade around 20.5 sen, off around 8.9 percent in intraday trade.
