It is too early to give a final verdict on what caused the Russian plane to crash over the Sinai Peninsula in Egypt on Saturday, but the terror factor cannot be ignored, the U.S. said on Thursday.

The statement came from White House spokesman Josh Earnest soon after claims were made on the possibility of the involvement of ISIS militants in planting a bomb aboard the plane. Earnest added that he couldn’t discuss or disclose what U.S. intelligence investigations have arrived at so far.

“At this point, we have not made our own determination about what exactly led to this tragic incident,” Earnest stated. “At the same time, we can’t rule anything out, including the possibility of terrorist involvement.”

U.K. Prime Minister David Cameron claimed earlier on Thursday that it was “more likely than not” that a bomb was what brought the Metrojet’s flight KGL9268, which was mostly occupied by Russian vacationers, down - a scenario that Russian and Egyptian officials dismissed as premature speculation. Meanwhile, the U.K. government announced on Friday that flights to Sinai could resume service after they were suspended as a result of the incident.

Earnest also said that the U.S. government was considering the matter a top priority and reviewing if any supplementary security measures were required to make sure the act is not repeated. On Thursday, U.S. President Barack Obama told KIRO Radio’s Dave Ross that he thought there was a chance of a bomb planted inside the plane, and that the U.S. authorities were taking the case very seriously.

The Kremlin has denied the claims of a planted bomb in the plane.

“Any sort of version of what happened and the reasons for what happened can only be put forward by the investigation and we have not heard any announcements from the investigation yet,” Kremlin spokesman Dmitry Peskov told reporters, as quoted by Sky News .

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