Reuters/AFP/Cairo

The cause of a Russian plane crash in Egypt on Saturday is looking more like an explosion but it is not clear whether it was linked to fuel or engine trouble or a bomb, an Egyptian source close to the investigation of the black boxes said on Wednesday.

"It is believed to be an explosion but what kind is not clear. There is an examination of the sand at the crash site to try and determine if it was a bomb," the source told Reuters.

"There are forensic investigations underway at the crash site. That will help determine the cause, to see if traces of explosives are found."

Egypt has dismissed claims by Islamic State that it brought down the aircraft with 224 passengers and crew aboard.

The country's aviation ministry said earlier the cockpit voice recorder from the plane was partially damaged but that black box data had been extracted and validated and was being analysed.

Britain concerned

Britain said it was concerned Wednesday that a Russian airliner that crashed in Sinai may have been brought down by a bomb, prompting it to temporarily suspend flights from Egypt's Red Sea resort of Sharm el-Sheikh.

"While the investigation is still ongoing we cannot say categorically why the Russian jet crashed. But as more information has come to light, we have become concerned that the plane may well have been brought down by an explosive device," the office of Prime Minister David Cameron said in a statement.