 -- Green card holders, who are legal residents of the United States, fall under President Trump's executive order on immigration if they come from any of the seven Muslim-dominated countries from which immigration is temporarily banned, according to a senior administration official who spoke to ABC News.

People from these countries who hold green cards to live in the U.S. but who are currently outside of the country will need a waiver to re-enter the United States. The waivers will be issued on a case-by-case basis, the official told ABC News.

Green card holders currently in the U.S., who hail from any of the seven countries listed in Trump's order, will need approval before leaving the country or risk being detained while re-entering the country, the official said.

The order signed by Trump on Friday, which he said is aimed at protecting the nation from terrorists, suspends for 90 days immigration to the U.S. from seven Muslim-majority nations -- Syria, Yemen, Sudan, Somalia, Iraq, Iran and Libya. It also suspends for 120 days the entry of refugees into the U.S. and indefinitely bans Syrian refugees from coming into the country.The list of countries are: targeted countries includes Iraq, Iran, Libya, Somalia, Sudan, Syria, and Yemen, which all have Muslim-majority populations.

As to people with green cards who are currently traveling or "in transit" back to the United States, the official said that the order has a hardship exemption. The official said administrators are still working out how to define "in transit" and "hardship."

The Department of Homeland Security and the State Department are currently finalizing guidance that will be issued to airports and airlines regarding implementation of the order, the official added.

The official praised the work of the agencies and officials who put the order in place "quickly and professionally" as soon as it was issued and said all the relevant officials were briefed.

The official also said that the Trump team has been working on the order for "weeks," starting during the transition period.

The order is already complicating travel for some. At least 27 people have been detained in four U.S. airports since the president signed the action.

Among those were two Iraqis held at Kennedy International Airport in New York City, one of whom was released later Saturday and allowed to enter the U.S..

In addition, seven U.S.-bound migrants -- six from Iraq and one from Yemen -- were prevented from boarding a flight in Egypt that was bound for Kennedy Airport, The Associated Press reported. Officials said the seven, escorted by representatives of the United Nations' refugee agency, were stopped from boarding the plane after authorities at Cairo's airport contacted their counterparts at the airport in New York.

Meanwhile, some colleges – including Princeton University, Rochester Institute of Technology in New York State and Stanford University’s center for international students are advising students from any of the seven countries covered by the executive order to consider delaying travel outside the U.S.

In addition, Bloomberg News obtained a staff memo circulated by Google Chief Executive Officer Sundar Pichai in which he criticized Trump's action and suggested that employees who could be affected it return to the U.S.