A Hong Kong airline has apologised after it demanded a passenger take a pregnancy test to prove that she was not pregnant before boarding a flight from Hong Kong to the US territory of Northern Mariana Island in the Pacific.

Key points: Northern Mariana Island has become a popular destination for people hoping to get their children US citizenship

Northern Mariana Island has become a popular destination for people hoping to get their children US citizenship Ms Nishida was escorted by staff to a bathroom to take the pregnancy test before they allowed her to fly

Ms Nishida was escorted by staff to a bathroom to take the pregnancy test before they allowed her to fly Hong Kong Express Airways initially claimed it was a "helping to enforce" US immigration laws but has since apologised

Japanese citizen Midori Nishida, 25, was flying with Hong Kong Express Airways to the island of Saipan, which is part of the Northern Mariana Island, last November when staff required her to take a "fit to fly" assessment.

The island has become a popular destination for so-called "birth tourism", where foreign nationals give birth in the territory to ensure their babies become US citizens.

CNN reported that birth tourism has become so rampant in the US territory in recent years that more babies were born to tourists than its permanent residents.

According to the US Government's official website, there are "no rules prohibiting pregnant visitors from entering the United States" — however entering the country "to give birth" is prohibited.

US Customs and Border Protection officers can deny pregnant visitors entry if they believe they will not be able to pay medical costs, taking into consideration the child's due date and the length of time the visitor plans to spend in the country.

Ms Nishida, who grew up on the island of Saipan and still has family on the US territory, filled in a questionnaire before her flight showing that she was not pregnant but was still asked to undergo the assessment and was escorted to a bathroom to take the test.

The woman was allowed to board the flight when the pregnancy test came back negative.

The airline initially claimed it was "helping to ensure US immigration laws were not being undermined" but since has apologised, NBC News reported.

In a statement provided to NBC News, the airline said it "would like to apologise unreservedly to anyone who has been affected by this".

"We took actions on flights to Saipan from February 2019 to help ensure US immigration laws were not being undermined," the airline, which was acquired in July by Cathay Pacific, said in the statement.

"We have immediately suspended the practice while we review it."

The company also said: "Under our new management, we recognise the significant concerns this practice has caused."

Ms Nishida told the Wall Street Journal that it "was very humiliating and frustrating" and the policy discriminated against women based on their appearance.

She told NBC News that following the incident she contacted Hong Kong Express Airways multiple times to file a complaint and request an explanation.

Ms Nishida received an apology via email from the airline after The Wall Street Journal contacted the airline for comment for its story.