The medical charity Doctors Without Borders has warned some mandatory US state Ebola quarantine measures are having a "chilling effect" on its work.

The group has said it may shorten some assignments to West Africa as a result of recent state restrictions.

One of the charity's volunteers has defied orders by the US state of Maine that she remain quarantined in her house after being in Sierra Leone.

There have been nearly 14,000 cases worldwide, but only nine in the US.

Doctors Without Borders - also known as Medecins Sans Frontieres (MSF) - has 270 international and 3,000 locally hired staff in Guinea, Liberia, and Sierra Leone.

But the foreign workers now have additional concerns when heading home, said executive director Sophie Delaunay.

"There is rising anxiety and confusion among staff members in the field over what they may face when they return home upon completion of their assignments in West Africa," she told Reuters news agency.

Some health workers are delaying returning to the US and staying in Europe for 21 days, she added, "in order to avoid facing rising stigmatisation at home and possible quarantine".

Some people are being discouraged by their families from returning to the field, she added.

In other developments:

• a UK ship has arrived in Sierra Leone carrying food, medical equipment and 32 pick-up trucks, to help keep hard-pressed Ebola treatment centres in operation
• speaking in Brussels after a trip to West Africa, US Ambassador to the UN Samantha Power has said the world must do more to confront "the greatest public health crisis ever"
• North Korea has instituted a 21-day quarantine for any foreign national arriving from any country
• the World Bank said it would immediately provide $100m to fund the deployment of more health workers to West Africa

Lawyers for Kaci Hickox, a nurse recently returned to the US from treating Ebola patients in Africa, have vowed to fight a court order that would enforce a 21-day quarantine.

Maine Governor Paul LePage said the state was willing to agree to arrangements that would have allowed Hickox to go for walks, runs and bicycle rides, but not allow her to go to public places.

The governor said discussions with Ms Hickox, 33, had failed.

She says her freedom should not be limited when she is perfectly healthy.

People are not infectious until they show symptoms, usually a fever.

Another worker, Dr Craig Spencer, travelled around New York City before he fell ill. He is currently in isolation in hospital.

After his case was announced, New York, New Jersey and other states ordered the mandatory quarantine of healthcare workers who had been exposed to Ebola patients.

But President Barack Obama has warned that overly restrictive measures could discourage volunteering in West Africa.

UN Secretary-General Ban Ki-moon has condemned the actions of US states ordering medics to be isolated.

"Returning health workers are exceptional people who are giving of themselves for humanity," he said.

"They should not be subjected to restrictions that are not based on science."

(CNN)