Mongolian immigration officers last week raided the giant copper-gold mine on suspicions workers at the project, which is a joint venture between Rio and the local government, did not have the correct visas.

About 40 workers, including Australians, Americans and Canadians, had their passports confiscated last Friday and were unable to return home on flights scheduled for Monday.

Since then, most of the workers were found to be working illegally and have been told to get out of the country.

The stoush in one of the most inhospitable places on earth was part of a wider crackdown by Mongolia on ex-pats working in the country.

Rio said the affected workers were employed by contractors, not directly by the mining giant.

“We are aware immigration officials in Mongolia have conducted visa checks at a number of businesses,” a company spokesman said.

“The responsibility for ensuring the correct visa has been applied for and issued sits with the contracting company. If found to hold an incorrect visa they may be asked to pay fines and leave the country in a matter of days, consistent with immigration law in many countries, including Australia.

“We are working closely with the contracting companies, the contractors, who remain on site, and the Mongolian immigration department as they work to complete their review. The department has been working through this in a timely manner. We expect it to be resolved quickly.”

Hofmann Engineering managing director Erich Hofmann would not comment on the situation other than to say the company was working to ensure its staff could get home as soon as possible.

A Department of Foreign Affairs and Trade spokesman said Canberra was “aware of the situation and is making inquiries with the relevant Mongolian authorities”.

Oyu Tolgoi is one of the most remote mine sites in the world. The sprawling project is 550km south of Mongolia’s capital, Ulaanbaatar and 80km north of the China border.