A doctor taking part in a widely-publicized polio immunization campaign is in a Pakistani hospital after being wounded when his vehicle came under heavy gunfire in the southern port city of Karachi.

Police say the foreign doctor and his driver were on their way to an immunization camp Tuesday when two men began shooting. The driver was also injured.

The nationality of the doctor was not immediately clear.

Police and the World Health Organization say it is unclear whether the vehicle was the target of the attack or simply caught in crossfire. Karachi has seen relentless ethnic, political and religious violence in recent months.

Threats from militants have hampered Pakistan's efforts to vaccinate children against polio, especially in the country's northwest.

On Friday, the social affairs secretary for Pakistan's Federally Administered Tribal Areas, Aftab Akbar Durrani, told VOA's Urdu Service some 250,000 children are at risk of not getting vaccinated during a three-day national anti-polio campaign that concludes Wednesday.

The tribal agencies that border Afghanistan are a known hub for Taliban and al-Qaida-linked militants.

Last month, a militant group based in North Waziristan and led by Hafiz Gul Bahadur warned anti-polio medical teams to stay away from the tribal agency or “face consequences.” Militants allege the effort is being used to gather information for U.S. drone strikes in the region.

Pakistan is one of only three countries in the world where polio is still prevalent. The other two nations are Afghanistan and Nigeria.

Polio usually infects children in unsanitary areas. The viral disease attacks the central nervous system, sometimes causing paralysis, muscular atrophy, deformation and death.