(CNN) -- Pakistani President Asif Ali Zardari condemned the Sunday killings of two polio vaccination volunteers in the country's northwest, calling their slayings "cowardly and inhuman."
The workers were shot to death while visiting homes in Khyber Pakhtunkhwa province in northwestern Pakistan, police said. Islamic militants have targeted anti-polio campaigns since U.S. intelligence used a fake vaccination program to aid its hunt for al Qaeda leader Osama bin Laden.
Pakistan is one of only three countries where polio remains endemic, with 58 cases reported in 2012, according to the Global Polio Eradication Initiative. In a statement issued by his office, Zardari said the volunteers died in service of a "noble cause" and called for tighter security for the effort.
"While condemning the death of the two polio volunteers, the president said that such cowardly and inhuman acts of the militants and extremists can not deter the strong resolve of government to eradicate polio from the country," the statement said.
The CIA used a fake vaccine program to collect DNA from residents of bin Laden's compound to verify the al Qaeda leader's presence before a May 2011 raid that killed him. Bin Laden had been holed up in the Pakistani city of Abbottabad, about 60 miles north of the capital, Islamabad.
Sunday's killings capped a bloody weekend dominated by a bus bombing and hospital siege in Quetta, the capital of the southwestern province of Balochistan. Militants attacked a university bus carrying women, then struck the hospital where the survivors were taken for treatment, authorities said Saturday.
The 28 dead included 14 women killed on the bus, along with four nurses, four police officers and four militants, police said. At one point, the attackers held hundreds of patients, physicians and nurses hostage as they battled security forces.
Earlier Saturday, assailants bombed the home of Muhammad Ali Jinnah, the founder of the Pakistani state, in Balochistan, killing a police officer and wounding an employee, authorities said. Interior Minister Chaudry Nisar said Sunday that there was a "secret war" going on in the province, and the government had to work "with all the parties involved" to bring it to an end.
"The government is ready to hold talks with those that believe in dialogue," he said. "But the government will not hold talks with those that propagate and practice extremism."
CNN's Aliza Kassim contributed to this report.