Warsaw (AFP) - Polish villagers got a surprise when half a dozen US military helicopters emerged from heavy fog to make an emergency landing in a rapeseed field.

Onlookers responded "first with worry and bewilderment, then curiosity," Stanislaw Raginiak from the office of the rural county of Gruta said Wednesday.

The five Black Hawk helicopters and one Chinook heavy transport helicopter touched down on Tuesday, while taking part in ongoing NATO military exercises in Poland and the Baltic states.

NATO has beefed up its security measures in the region to reassure residents concerned over potential spillover from the escalating crisis in neighbouring Ukraine.

"Once they stepped off the aircraft, the crews explained they were forced to make the emergency landing because of bad weather," Raginiak told AFP.

"We chatted with them. They were from California, Michigan, Idaho."

Around 40 villagers, including local officials, greeted the surprise visitors, who turned out to be friends not foes.

"They invited us to tour their aircraft, which were really impressive. I got the chance to sit in the pilot's seat," said county office intern Daniel Zurawski.

He said the helicopters took off two hours later, kicking up clouds of dust.

Local media said the emergency landing caused the helicopters to go off the radar, setting off the alert systems of emergency services and military police.

Search teams grappled to locate the missing choppers, initially scouring an area 20 kilometres (12 miles) away.