Half a million American citizens could find themselves barred from returning home if they hold joint passports with one of seven countries whose nationals President Donald Trump has temporarily blocked from entering the US.

The executive order signed by Mr Trump barred newcomers from the seven “terror-prone” nations including Iran, Iraq, Libya, Somalia, Sudan and Yemen – but could affect hundreds of thousands of people from these countries who already live in the US under green cards or temporary student or employment visas.

Homeland Security confirmed the travel ban applies to those with current visas or even green cards from returning to the US from trips abroad.

Several people with valid visas at airports have already been held, including Hameed Khalid Darweesh, an Iraqi citizeen who worked as a US army translator. He said he would be killed if he was forced to return to his homeland.

After being detained on arrival for 17 hours, Mr Darweesh wept as he told reporters the incident hadn't changed his love of the US: ““America is the land of freedom, the land of the life. This is why I came here. Really, I am very thankful.”

Mr Trump blocked entry for all refugees for 120 days and for all citizens from the seven named countries for 90 days - although it specifically exempts from the ban those who hold certain diplomatic visas.

Donald Trump announces a ban on refugees and all visitors from Muslim-majority countries

Not included in the exemption, however, are those who hold long-term temporary visas — such as students or employees — who have the right to live in the US for years at a time, as well as to travel abroad and back at will.

The travel ban has received widespread condemnation, although many of his supporters in the US have welcomed the move – something Mr Trump indicated he would do during his campaign.

Around 25,000 citizens from the seven countries specified in Mr Trump’s order have been issued student or employment visas in the past three years, according to Department of Homeland Security reports.

Almost 500,000 people from the seven countries have received green cards in the past decade, allowing them to live and work in the US indefinitely.

Legally, green card holders remain classed as “aliens” in the US. While lawyers are unsure if they would actually be barred from returning to the US if they have travelled abroad, many concede it is a possibility.

Citizens of Iran and Iraq far outnumber those from the other five, less affluent countries among green card and visa holders. In the past 10 years, Iranian and Iraqi citizens have received over 250,000 green cards.

The National Council for Iranian Americans have called for a “grace period” on the “inhumane Trump ban”.

It claimed that it is receiving growing numbers of reports of visa holders and lawful permanent residents being detained at US airports.

“The horrific nature of Trump’s Muslim ban is fully apparent for the entire world to see," it said. “Families are being ripped apart without warning and with no assurance of when they will be reunited. Students travelling abroad at the time of the ban are horrified that they might not be able to return to continue their studies. Children are being detained along with their parents when they were just seeking to return home. This is a dark day in the history of this country.”

It added: “If the Trump administration has any shred of humanity and decency, it will institute a grace period without delay in order to enable all lawful permanent residents, dual nationals and visa holders from Iran and the other targeted countries to return to the United States to reunite with their families and return to their daily lives. Anything less is a complete disgrace.”

British citizens who have dual citizenship from one of the seven barred nations will also be prevented from entering the US for the next 90 days.