WASHINGTON/CHICAGO (REUTERS) - Mandatory quarantines for travellers from Ebola-affected West African countries came under fire on Sunday from the White House and a top U.S. health official, while a nurse who became the first health worker isolated under the new measures plans to file a lawsuit.

Kaci Hickox, being held in isolation at a Newark, New Jersey hospital, planned to contest her quarantine in court, her attorney said, arguing that the order violated her constitutional rights.

The nurse arrived on Friday at Newark Liberty International Airport after working in Sierra Leone with Ebola patients and was put into quarantine under New Jersey's 21-day mandatory quarantine.

New Jersey, New York and Illinois have began a policy of imposing quarantines on anyone arriving with a high risk of having contracted Ebola in Sierra Leone, Liberia and Guinea, where the epidemic has killed nearly 5,000 people.

The White House voiced its concern to the governors of New York and New Jersey about the potential impact of quarantine orders, a senior administration official said on Sunday.

"We have let the governors of New York, New Jersey, and others states know that we have concerns with the unintended consequences of policies not grounded in science may have on efforts to combat Ebola at its source in West Africa," the Obama administration official said in a statement.

Hickox's confinement raises "serious constitutional and civil liberties issues," given that she remains asymptomatic and has not tested positive for Ebola, her attorney Norman Siegel, a prominent civil liberties lawyer, said on Sunday.

The new rules were imposed a day after a New York doctor, Craig Spencer, was diagnosed with Ebola on Thursday after he returned from treating patients in Guinea. The doctor, now in isolation, moved freely around the city before he had symptoms that would make him contagious.

Medical professionals note that Ebola is extremely difficult to catch. It is spread through direct contact with bodily fluids from an infected person and is not transmitted by people who are not showing symptoms.

Before the White House's statement, a senior U.S. health official described the new rules as "a bit draconian", saying they could discourage medical workers from helping fight the epidemic.

"The best way to stop this epidemic is to help the people in West Africa. We do that by sending people over there, not only from the U.S.A. but from other places," Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, told NBC's "Meet the Press."

Hickox herself slammed the quarantine policy in an interview, describing hours of questioning and transfer to a hospital isolation tent. She told CNN she was particularly angry about Christie's remarks that she was "obviously ill" when she arrived in Newark.

"First of all I don't think he's a doctor and secondly he's never laid eyes on me," she said.

"I am completely healthy and with no symptoms," Hickox told CNN.

But New Jersey's governor defended his approach.

"This is government's job. If anything else, the government's job is to protect the safety and health of our citizens,"Christie told the Fox News Sunday TV programme.

The virus has killed almost half of more than 10,000 people diagnosed with the disease - predominantly in Liberia, Sierra Leone and Guinea.

In defending the new quarantine rules, Christie is emerging as a prominent critic of the Obama administration's handling of the Ebola crisis.

The New Jersey governor is considered a likely candidate for the Republican presidential nomination, and his aggressive stance could give a boost to his party in the midterm elections on Nov. 4, when the party hopes to take control of the Senate.

Missteps in handling the first diagnosis of Ebola on U.S. soil - a Liberian visitor to Texas in September - have opened the door for Republican attacks on President Barack Obama and his administration.

Even so, only four people, including the Liberian who died in Dallas, have been diagnosed with Ebola in the United States.

Spencer and Hickox worked with Doctors Without Borders. The charity, a leading part of international efforts to fight the epidemic.

Newark's University Hospital said the nurse remained in isolation and under observation "in a climate-controlled, indoor, extended care area in a building adjacent to the hospital". Doctors Without Borders said her tent was not heated.

Samantha Power, the U.S. Ambassador to the United Nations, arrived in Guinea's capital Conakry on Sunday and planned also to visit Liberia and Sierra Leone for a first-hand view of the global response to Ebola.