London, November 5 (RHC)-- Britain says an "explosive device" may have brought down a Russian passenger plane that crashed in Egypt's Sinai Peninsula last week, killing all the 224 people on board.

"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," said a spokeswoman for British Prime Minister David Cameron's office on Wednesday.

The British government has also put all flights to the UK from the Egyptian resort town of Sharm el-Sheikh on hold as a "precautionary measure," she added.

The spokeswoman noted that the moratorium was put in place so that a team of UK aviation experts could arrive at the resort town and asses security measures implemented at the airport.

"We expect this assessment to be completed tonight," she said, adding: "We have deployed extra consular staff to Sharm who will be on hand at the airport, working with the airlines, to assist British holidaymakers there."

The announcement followed an Egyptian government statement that stressed further analysis was required before conclusions could be made on the crash.

Also on Wednesday, the North African country's civil aviation ministry announced that the plane's second black box had been found.

"The cockpit voice recorder (CVR) is partially damaged and a lot of work is required in order to extract the data from it. Consequently, no further comment on the content of the CVR can be made. Examination of parts on site is continuing," read the statement.

Meanwhile, a U.S. intelligence official familiar with the subject also said the blast was probably caused by a bomb planted on the plane. "There is a definite feeling it was an explosive device planted in luggage or somewhere on the plane," CNN quoted the unnamed official as saying on Wednesday.

Russia and Egypt have so far dismissed suggestions that the plane crash was linked to terrorism.

The Airbus A321, run by Russia's Kogalymavia airline, dropped off radar screens on Saturday, 23 minutes after takeoff from Sharm el-Sheikh.