Declan Walsh / New York Times News Service

ISLAMABAD — Gunmen opened fire on a U.N. vehicle in the southern port city of Karachi on Tuesday, seriously wounding a Ghanaian doctor who was part of an urgent campaign to eradicate polio from Pakistan.

The attack was a further blow to the three-day polio vaccination drive, which had already been stymied in some parts of the country by Taliban threats.

The U.N. vehicle came under fire as it passed through Gadap, a neighborhood on the northwestern outskirts of Karachi, early Tuesday. The doctor, who was shot in the abdomen, was taken to the private Aga Khan Hospital for emergency treatment, according to Guido Sabatinelli, country director for the World Health Organization, who spoke by phone from Karachi.

The doctor’s driver, who was less seriously injured, was also treated and released, Sabatinelli said. He said he could not release the victims’ names because his staff had not yet reached the doctor’s family in Ghana.

The United Nations said its security officials were investigating the shooting together with the Pakistani police.

“We don’t know exactly why this shooting took place, but it is probably related to the polio eradication program,” Sabatinelli said.

The shooting cast an ominous cloud over a major effort to push back a rising tide of polio cases in Pakistan, one of just three countries where the disease is still endemic; the others are Afghanistan and Nigeria. Pakistan reported 198 new infections in 2011.

Thousands of Pakistani government health workers are walking door to door, administering oral vaccines to children under 5. The United Nations, the Bill and Melinda Gates Foundation and other private donors are helping to finance the effort and providing expert assistance.

The campaign is difficult because every vulnerable child must be vaccinated several times to be fully protected, which is especially difficult to achieve in the violent northwestern tribal belt along the Afghan border, the last major reservoir of new infections in the country.

It was not clear why the Ghanaian doctor came under attack in Karachi, which has also seen a spike in polio cases, caused in part by an influx of ethnic Pashtuns fleeing fighting in the northwest. Many of the Pashtuns in the city live in cramped, impoverished conditions, which can foster the spread of the disease, particularly in the humid summer.