Technically it is impossible, but theoretically everything is possible

Ukrainian President Leonid Kuchma

Ukraine's President, Leonid Kuchma, said that it was "technically an impossibility" that a Ukrainian missile had caused the crash.

But, taking a softer stance than he did immediately after the crash, he said "all theories should be discussed".

The head of Ukraine's air defences, Volodymyr Tkachov, said two missiles had been fired near the time of the crash on Thursday.

He said one of these hit its target, while the other, which missed, landed in the Black Sea 80 kilometres (50 miles) from the launch site on the Crimean peninsula before the crash occurred.

Officials had earlier insisted the missile travelled only 40km (25 miles). The aircraft plunged into the sea at a distance of about 250km (156 miles) from land with the loss of the crew and all 78 passengers.

Cover-up accusations

"We are interested in learning the truth as never before, no matter what the truth might be," Mr Kuchma said.

Relatives of the victims have accused the Ukrainian Government of trying to cover up the cause of the crash.

All the data we have show that the Ukrainian anti-aircraft corps were not involved in this accident

Head of Ukraine Air Defences Volodymyr Tkachov

"Technically it is impossible [that a missile hit the plane], but theoretically everything is possible," said Mr Kuchma.

"Following a catastrophe all theories should be discussed but only by experts," he added.

The United States maintains there is "every indication" a stray Ukrainian missile downed the plane, which was travelling from Israel to Novosibirsk.

Other possible explanations for the crash under consideration are a terrorist act or a mechanical failure.

Missile tracked

"All the data we have show that the Ukrainian anti-aircraft corps were not involved in this accident," Mr Tkachov told a news conference in the Ukrainian capital, Kiev.

He released video recordings of the two missiles' trajectories and traced their flight paths on a map.

Russian investigations focus on the terrorist theory

He said the launch time of the other 21 missiles fired during the exercises did not coincide with the time of the crash.

Mr Tkachov's announcement comes after Russia said it was not satisfied with the information provided by Ukraine.

Ukraine has joined Israel in promising to help in Russia's investigations and sent a delegation to the Russian city of Sochi to join the Russian team.

Gathering evidence

Russia - which says a missile was fired four minutes before the crash - has also asked the US to provide documentary evidence that it was a missile that downed the plane.

The US tracked a missile during the Ukrainian military exercise with satellites that sense the heat of its launch. US officials also said the timing of the launch coincided with the disaster.

Russia says "foreign objects" recovered during the salvage operation could not have come from the plane itself.

Experts deciphering radio transmissions also claim to have identified the time of the crash after they heard the pilot cry out shortly before the plane exploded.

Some observers say these pieces of evidence indicate a missile attack, but others say they support the terrorist theory.