Immigration agents have been wrongly telling travellers arriving legally in the US that they must sign documents forfeiting their right to enter, according to lawyers and migrants.

People holding valid visas were reportedly told to sign a form before being marched back onto flights to countries they had travelled from.

Others refused to sign and were held in detention for hours or days, before being allowed into the country, reports the New York Times.

It comes after Donald Trump signed an executive order on Friday banning people travelling to the US from seven Muslim-majority countries.

Lawyers told the newspaper that some immigration agents did not explain the content of the documents they were telling their clients to sign, and threatened barring them for years if they refused.

The people affected are all from countries whose citizens are subject to the 90-day ban: Iran, Iraq, Libya, Somalia, Sudan, Syria and Yemen.

Sara Yarjani, an Iranian student doing a masters degree, said she flew back to California after visiting her parents in Austria.

James Corden films airport journey in protest at Donald Trump's 'Muslim ban'

At Los Angeles International Airport she claims she was questioned, searched, and put in detention for 23 hours before being told she could not enter the country because her visa had expired.

Ms Yarjani told the New York Times an officer said: “Either you comply and agree to leave voluntarily, by signing a document, or if you do not comply we will have to forcibly deport you, which will face you with a re-entry ban to the United States of one to five years or longer.”

Another Iranian student, known only as Saba, told the newspaper she was returning from Europe when an immigration officer said she could not enter. Her visa expires in 2018.

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She claims an officer said to sign a document without knowing what it stated. When she was given a copy, she was told she had voluntarily withdrawn her application to enter the US.

There is still confusion over whether Mr Trump’s banning order applies to people with temporary visas.