CNN  — 

Two polio workers were killed in an attack in a tribal area of Pakistan on Saturday, according to a local official.

The workers, both men, were killed during inspections when their team of four came under fire by unknown assailants, Mehmood Aslman, the political agent for Mohmand Agency, told CNN.

Mohmand Agency, a district in Pakistan’s Federally Administered Tribal Areas, borders Afghanistan in the country’s mountainous northwest region. Because of communication difficulties in the area, authorities did not learn of the attack until Sunday morning.

No groups have come forward to claim responsibility for the attack.

Pakistan, Afghanistan and Nigeria are the only countries in the world where polio is endemic, according to the World Health Organization and the Global Polio Eradication Initiative.

Health workers in Pakistan vaccinating children against polio have come under attack in recent years by militants, who have targeted anti-polo campaigns since US intelligence officials used a fake vaccination program as part of the hunt for Osama bin Laden in 2011.

Under the guise of the fake campaign, the CIA sought to collect DNA samples from bin Laden’s relatives to verify his presence at a compound in Abbottabad, Pakistan.

Attacks on polio workers or police escorts tasked with protecting them were frequent for several years but had recently slowed due to an ongoing military operation.

CNN’s Richard Greene contributed to this report.