LONDON (Reuters) - One in five African-born doctors work abroad in developed countries, according to a study highlighting the exodus of physicians and nurses critical to a region struggling with a worsening health crisis.

The U.S. researchers used census data on arriving African health professionals to nine major destination countries and said the numbers have increased since they carried out the survey between 1999 and 2001.

The reasons are clear, said Michael Clemens of the Center for Global Development in Washington D.C., who led the study published on Thursday.

"A Kenyan nurse working in London isn't taking care of sick people in Kenya, but that nurse is pursuing professional possibilities that aren't available to her at home," Clemens said.

While emigration is not a new phenomenon, its acceleration since the independence era of the 1960s has hit the health sector the hardest with conflict, poverty and instability driving many to work in richer countries.

In their study, the researchers estimated that about 135,000 African-born physicians and professional nurses practice overseas in developed countries. This works out to about one-fifth of the doctors and 10 percent of the nurses.

Yet the numbers vary greatly, with recent civil wars or economic woes playing big factors, the researchers said.

"Angola, Congo-Brazzaville (Republic of Congo), Guinea-Bissau, Liberia, Mozambique, Rwanda, and Sierra Leone all experienced civil war in the 1990s and all had lost more than 40 percent of their physicians by 2000," the researchers wrote in the journal Human Resources for Health.

"Kenya, Tanzania and Zimbabwe all experienced decades of economic stagnation in the late 20th century and by its end, each had lost more than half its physicians," they added.

At the same time more stable and prosperous countries such as Botswana, South Africa and Ivory Coast -- before it slipped into civil war -- managed to keep their doctors.