A Ukrainian official today admitted that his country's military may have mistakenly shot down a Russian commercial airliner over the Black sea last week, killing all 78 people on board.

The announcement by Yevhen Marchuk, the head of the Ukrainian security council and a member of the commission investigating the tragedy, marks the first time that the country has conceded it may be responsible.

"The reason for the crash could be an unintentional hit by an S-200 missile during the Ukrainian air defence exercises," Mr Marchuk told a press conference today, adding that investigators would make their final conclusions after further, complex research.

The Russian chief of the investigative commission, Vladimir Rushailo, said today that the aircraft had been hit by an anti-aircraft missile, the Interfax news agency reported.

Mr Rushailo said: "The Tu-154 flying from Tel Aviv to Novosibirsk crashed because it was hit by the warhead of an anti-aircraft missile." He added that investigators were continuing to examine fragments of the downed plane.

The Russian Tu-154 airliner crashed October 4 off the Black Sea coast, near the Russian city of Sochi, after an explosion on board. All 78 crew and passengers, most of them recent Russian immigrants to Israel, were killed.

Hours after the crash, US officials said that the tragedy had been caused by an S-200 missile fired mistakenly by Ukrainian forces during military exercises on the Crimean peninsula, which juts into the Black sea.

Pressure has been mounting on Ukraine's leadership to take responsibility for the crash. Ukrainian military officials have heatedly denied they were at fault, but the Ukrainian president, Leonid Kuchma, said yesterday that the defence minister, Oleksandr Kuzmuk, had submitted his resignation immediately after the tragedy. The resignation was rejected.

Mr Kuchma today ordered the creation of a Ukrainian commission to look into the causes of the tragedy, together with Russia.

Russian investigators had initially focused on the possibility of a terrorist attack, but officials signalled over the weekend that they were considering the Ukrainian missile theory ever more likely.

On Tuesday, a top investigator, former Soviet air force chief Yevgeny Shaposhnikov, said experts had found fragments resembling the missile's payload at the Black sea crash site.