CAIRO (Reuters) - Investigators have extracted and validated the contents of the flight data recorder, one of two so-called black boxes recovered from the Russian airplane that crashed in Egypt, Egypt's Civil Aviation Ministry said on Wednesday.

The ministry said the second black box which contains the cockpit voice recorder (CVR) was partially damaged and much work was required to extract data from it.

"Consequently no further comment on the CVR can be made. Examination of parts on site is continuing," said the ministry's statement.

Earlier, Islamic State's Egyptian affiliate dismissed in an audio message doubts that it had downed the Russian passenger plane over Egypt's Sinai Peninsula, killing all aboard, and said it would tell the world how it did so in its own time.

The Russian-operated Airbus A321M crashed on Saturday shortly after taking off from the Red Sea resort of Sharm al-Sheikh on its way to St Petersburg, killing all 224 people on board.

Sinai Province, an Egyptian group loyal to Islamic State, said in a statement the same day that it had brought down the airliner "in response to Russian air strikes that killed hundreds of Muslims on Syrian land".

The claim was dismissed by Russian and Egyptian officials. Security experts and investigators have said the plane is unlikely to have been struck from the outside and Sinai-based militants are not believed to possess the technology to shoot down a jet from a cruising altitude above 30,000 feet.

Russian officials have, however, said the plane probably broke up in the air, leaving open the prospect of some kind of explosion on board.

Asked to comment on those remarks and local press reports that the black box voice recorders had picked up unusual sounds before the crash, Civil Aviation Minister Hossam Kemal said the facts had yet to be established.

"This is all speculation. There is nothing definitive until the investigation commission completes its probe," he said.

In an audio message posted on a Twitter account used by the group, Islamic State's Egyptian affiliate insisted it was behind the crash. The claim could not immediately be authenticated.

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"We, with God's grace, are the ones who brought it down, and we are not obliged to disclose the mechanism of its demise," the speaker said.

Russia, an ally of Syrian President Bashar al-Assad, launched air raids against opposition groups in Syria including Islamic State on Sept. 30. The hardline group has called for war against both Russia and the United States in response to their air strikes in Syria.

Islamic State backers in Iraq issued a video on Tuesday congratulating their Egyptian colleagues and warning Russian President Vladimir Putin that more was to come.

Sinai-based militants have killed hundreds of Egyptian soldiers and police in recent years and have also attacked Western targets. Egypt has carried out air strikes on them.

Islamic State websites have in the past claimed responsibility for actions that have not been conclusively attributed to them. Officials say there is no evidence to suggest so far that a bomb brought down the plane.

(Reporting by Lin Noueihed; Editing by Michael Georgy)