Gunmen have opened fire on a World Health Organization team working on a polio immunization campaign in southern Pakistan, wounding two people.

WHO says a staff member and an international consultant are in stable condition at a hospital, after Tuesday's attack in the port city of Karachi. Pakistani police had earlier identified the two as a foreign doctor and a driver. Their nationalities were not immediately known.

Both men were supporting a three-day national polio immunization campaign that began on Monday. WHO issued a statement, saying “at this point, there is no evidence to suggest that this was a deliberate or targeted attack against polio eradication efforts or WHO.”

The U.N. agency noted “incidents like these highlight the incredible bravery of the more than 200,000 mainly Pakistani volunteers who run every vaccination campaign.”

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.