The US will step up security precautions on some US-bound flights from the Middle East as investigators determine what caused a Russian airliner to crash in Egypt over the weekend.

US Department of Homeland Security Secretary Jeh Johnson said in a statement that "out of abundance of caution" the US would expand screening on aircraft coming from the Middle East, conduct airport assessments in the region, and offer airport security and aviation assistance to certain airports. The US will also take other steps "both seen and unseen," he said.

Johnson said that the enhancements are "designed to provide an additional layer of security" on top of security measures that are already in place on all US-bound flights from the region.

The announcement comes as there is mounting evidence the Russian carrier Metrojet's Airbus A321-200 was brought down by an explosion nearly 20 minutes after take off from the resort town of Sharm el-Sheikh in the Sinai Pennisula, where an "Islamic State" (IS) offshoot is active.

IS claimed the attack shortly after the plane went down on Saturday, but Russian and Egyptian officials initially remained skeptical.

But on Friday Russia decided to halt all flights to Egypt until security was improved at airports. The move meant Russia followed the UK and Ireland in suspending flights to the Sinai resort, after both US and UK officials said their intelligence suggested the plane was likely brought down in a terrorist attack. The UK is trying to bring back thousands of stranded tourists amid confusion and delay.

Some airlines including Lufthansa have also stopped flying to the resort town. There are no direct flights between Sharm el-Sheikh and the United States.