LOS POTOCOS, Venezuela (AP) _ Amid the rows of shacks inhabited by poor farmers and unemployed laborers, doctors in white coats now make house calls, dodging children running barefoot and chickens pecking at piles of trash.

Those doctors _ on loan from Cuba _ are a welcome sight in this dusty village and other forlorn corners of Venezuela where physicians seldom set foot.

Cuba's communist government is expanding a humanitarian mission that has already sent a fifth of the island's doctors to work in Venezuela, committing more aid to its close ally as Cuba receives massive shipments of Venezuelan oil.

``Thank God those doctors are here,'' said Maria Aray, 50, who said the Cubans helped her 14-year-old daughter recover from severe anemia.

The Venezuelan government says the program involves about 20,000 Cubans, including more than 14,000 physicians _ an estimated 20 percent of Cuba's doctors. Cuban President Fidel Castro has pledged to have up to 30,000 health care workers in Venezuela by the end of the year.

Castro has long sent doctors on missions to countries ranging from Haiti to Equatorial Guinea _ always treating the poor for free _ but Venezuela in recent years has become their top destination.

In turn, Venezuela ships Cuba 90,000 barrels of oil a day under preferential terms, a deal giving the island one of its strongest economic boosts since the fall of the Soviet Union. Venezuelan President Hugo Chavez says the medical mission is unrelated to the oil deal.

But so many doctors have gone to Venezuela that some Cubans complain health care on the island is suffering. Castro insists they are mistaken, and that there are enough doctors to go around. Both countries, he says, are reaping the benefits of cooperation.

Chavez, who describes Castro as his mentor, calls the program part of a ``war to save lives'' as Venezuela moves toward socialism, and he says the benefits are clear. In addition to the visiting doctors, thousands of Venezuelans have been flown to Cuba to undergo surgery for free.

In Los Potocos, 150 miles east of Caracas, the three Cuban doctors see their presence as vital for poor patients who can't afford even basic medicines.

``This is the first time I've left Cuba, and I've never seen anything like this,'' said Dr. Leonardo Hernandez, 27, checking the pulse of a 2-month-old boy in a home where wires dangled from light fixtures and concrete walls were covered in grime.

When he and his colleagues arrived two years ago, they found malnourished children and widespread diarrhea. Now, they say, vitamins are making the children healthier, and there have been vast improvements in sanitation.

Some critics accuse Chavez of using the Cubans to maintain political support while neglecting public hospitals that have outdated equipment and shortages of supplies from needles to surgical thread.

The government has pledged to fix or replace old hospitals, but some Venezuelan doctors say Chavez is instead creating a parallel health system. The Venezuelan Medical Federation has urged its members to protest in Caracas on Friday to demand pay raises and oppose the Cubans' ``illegal'' practice of medicine in the country.

``We don't need them at all. What we need is a coherent health policy,'' said the federation's president, Dr. Douglas Leon Natera.

Chavez has called the Cuban medical mission the best hope for improving care for the poor, and has urged more Venezuelan doctors to join. His government has begun training thousands of new doctors.

Some accuse Venezuelan doctors of treating the wealthy while ignoring the hillside slums of Caracas.

``Why don't they come up here into the hills?'' said Dr. Marta Diaz, a 43-year-old Cuban who for two years has been offering acupuncture along with traditional medicine.

Diaz said she would gladly stay as long as needed, even though it keeps her apart from her husband and two daughters.

Doctors who accept an invitation to work in Venezuela receive an extra stipend of $186 a month from the Venezuelan government, while Cuba continues to pay their families their regular salaries, commonly in the range of $25 a month.

Otto Sanchez, a Cuban doctor who defected in 2003, said he felt he was being used for a ``political program.''

``You realize that you're being exploited with the kind of salary you're paid,'' said Sanchez, 38, who complained doctors were given pamphlets to hand out praising Cuba's medical system.

Sanchez now lives in Miami, where he is part of a group that has helped more than 20 Cuban doctors desert their posts in Venezuela.

But Hernandez said he doesn't understand how doctors can leave when they are so badly needed.

``It's like betraying oneself as a doctor, as a person,'' Hernandez said. ``What we're doing here is something too beautiful to stop.''

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AP correspondent Andrea Rodriguez contributed to this report from Havana, Cuba.