US paratrooper Gregory Crocker is giving a group of Ukrainian soldiers lesson one in dealing with an unexploded bomb: don't touch it until the experts arrive.

In a mock-up of a house rigged with explosives, the Ukrainian trainees set off alarms when they fiddle with the fake devices -- part of their preparation for fighting pro-Russian separatists in the east.

"Don't neutralise the bomb!" Crocker cries, debriefing them afterwards.

"But..." replies one of his trainees. "It's in the Ukrainian mentality to do just that."

The US 173rd Airborne Brigade started on April 20 training troops of the Ukrainian National Guard at this military base in western Ukraine.

Among the 300 US paratroopers and 900 members of Ukraine's National Guard in Operation Fearless Guardian, there is laughter and camaraderie -- but also language barriers and mutual cultural bafflement.

Ideas differ on everything from how to handle explosives to how to hold a gun.

"We get on well with the Americans. We train together, we eat together, we play sports together in the mornings," says one 32-year-old Ukrainian soldier, who gave his name as Dmytro.

The training is tricky, however, as few of the Ukrainians speak English. Some of those serving as interpreters at times struggle to make sense of the discussions.

"Yesterday we had our first day of training and it was really difficult, and now it is (still) difficult," said one of them, Artem Matza, a 19-year-old National Guard cadet.

"It is hard to understand the Americans because they speak so fast," he added.

"But I think that when we have had one week of training it will be simpler," he added. "We just need practice, that's all."

- Ukrainian-American soldiers -

The US army has brought some of its own interpreters: around 20 of its soldiers on the mission are of Ukrainian origin.

"I was born in Ukraine and lived part of my life here, so I understand both sides very well," said one US soldier, Anton Klokun, in Ukrainian.

He has lived in the United States for the past seven years and joined the US army two years ago.

"I think I am helping both countries this way, and I am delighted to do so," he said.

Another member of the US contingent, Oleksandr Skripnichuk, reckons if he hadn't moved to the United States when he was 10, he would be in the Ukrainian army now.

"I am sorry that I cannot serve the Ukrainian forces, but I am already enlisted in the US army. I fight for America," he said.

"But I am happy that I can come here on this mission and help the Ukrainians defend their country."

- 'Away from the damn bomb' -

A regular force under the control of Ukraine's interior ministry, the National Guard has absorbed militia groups that took part in last year's uprising against pro-Russian former president Viktor Yanukovych.

Many of them lack any formal military training.

Yanukovych's ousting was followed by uprisings by pro-Russians in the east. Fighting between those forces and Ukraine's military has killed more than 6,000 people in the past year.

The US training mission has angered Moscow, which warned it could "destabilise" the situation in Ukraine.

Laughter breaks out as Crocker labours for 10 minutes trying to explain that no one without bomb-defusing training must touch an unexploded device.

He sends his trainees on a fresh simulation exercise in the house supposedly rigged with explosives.

Gradually he appears to win over the sceptical Ukrainians.

As the real conflict continues, "the bombs are going to get better, they will get more dangerous," he warns.

"So get away from the damn bomb."