Muslim travellers have reported being prevented from boarding flights and turned away at US border control after Donald Trump signed an executive order banning citizens of seven Middle Eastern countries from entering the United States.

Mr Trump signed an executive order closing US borders to all refugees for a period of at least four months and temporarily banning all travellers from half a dozen countries, regardless of whether they have already been issued visas, on Friday evening.

“We want to ensure that we are not admitting into our country the very threats our soldiers are fighting overseas,” Trump said as he signed the order at the Pentagon. “We only want to admit those into our country who will support our country and love deeply our people.”

The order, which came into force as soon as Mr Trump signed it, requires US border officials to turn away any traveller from Syria, Iraq, Iran, Sudan, Libya, Somalia and Yemen for the next 90 days.

With only a few exceptions for diplomats and dual citizens, the order takes no account of whether travellers have already been issued with visas or have visited the United States before.

There were immediate reports of travellers who had been issued visas for travel being turned away or told not to board flights because of the ban.

Mohammed Al Rawi, a graduate of California State University and former journalist with the Los Angeles Times, said his father had been hauled off a flight in Qatar as a direct result of Mr Trump’s decision.

“My 71 year old dad is in Qatar boarding LAX flight to come visit us and and he’s being sent back to Iraq. Some US official told him that Trump canceled all visas,” he wrote on Facebook hours after the order was signed.

Five Iraqi passengers and one Yemeni were barred from boarding an EgyptAir flight from Cairo to New York on Saturday.

The passengers, arriving in transit to Cairo airport, were stopped and re-directed to flights headed for their home countries despite holding valid visas, Reuters reported.

Some airlines have warned that all passengers whose journeys began in any of the seven countries may be affected, even if their own citizenship is not on the “banned” list.

Vera Mironova, a Russian citizen returning from an academic research trip to Iraq, said she had been warned at check in that she may not be allowed into the US despite holding a green card.

“I just talked to Lufthansa guys and since an hour ago they need to inform all people traveling from Iraq about this possibility,” she said before boarding on Saturday afternoon.

Google recalled all travelling staff members to the United States following the order amid concern about the possible impact on recruiting top talent abroad.

A Google spokesperson said: “We’re concerned about the impact of this order and any proposals that could impose restrictions on Googlers and their families, or that could create barriers to bringing great talent to the US.  We’ll continue to make our views on these issues known to leaders in Washington and elsewhere.”

Mark Zuckerberg, the founder of Facebook, wrote in a post on Saturday that he was “concerned” about the impact of the orders and that he would be working with Fwd.US, a charity he supports, to develop protections for child immigrants brought to the US at a young age by their parents.

The order signed by Mr Trump also imposes a 120-day suspension the US refugee resettlement programme, regardless of applicants’ country of origin, while administration officials develop additional vetting procedures and decide which countries those procedures are “adequate” to ensure safety.

Syrian refugees are singled out as “detrimental to the interests of the United States” and banned from entering the country indefinitely.

The U.S. may admit refugees on a case-by-case basis during the freeze, and the government will continue to process requests from people claiming religious persecution, “provided that the religion of the individual is a minority religion in the individual’s country.”

The order suspended a resettlement programme that allowed 85,000 people fleeing war, hunger, and political or religious persecution, to be resettled in the US last year.

Paul Ryan, the Republican speaker of the House, said it was “time to re-evaluate and strengthen the visa-vetting process.”

Many Democrats decried the move as “un-American.”

“Tears are running down the cheeks of the Statue of Liberty tonight as a grand tradition of America, welcoming immigrants, that has existed since America was founded has been stomped upon,” said Chuck Schumer, Senate Minority Leader.

The United Nations’ refugee agency (UNHCR) and International Organization for Migration (IOM) called on Donald Trump’s administration to continue offering asylum to people fleeing war and persecution, saying its resettlement programme was vital.

“The needs of refugees and migrants worldwide have never been greater and the U.S. resettlement programme is one of the most important in the world,” the two Geneva-based agencies said in a joint statement.

“We strongly believe that refugees should receive equal treatment for protection and assistance, and opportunities for resettlement, regardless of their religion, nationality or race,” they said.

The agencies hoped “that the US will continue its strong leadership role and long tradition of protecting those who are fleeing conflict and persecution”.

Some 25,000 refugees were resettled in the United States between October and year-end under UNHCR’s programme for the most vulnerable, the agency said on Friday.

The Council on American-Islamic Relations (CAIR) said it would file a federal lawsuit challenging the constitutionality of the executive order on Monday.

“There is no evidence that refugees – the most thoroughly vetted of all people entering our nation – are a threat to national security,” said Lena Masri, the group’s director of National Litigation.