The attack followed violence this weekend that killed four U.S. troops and four Afghan soldiers.
Wednesday's attack occurred in Khair Khana, a village in Badghis province, 340 miles west of the capital, Kabul, provincial police chief Amir Shah Naibzada told The Associated Press.
An official for international aid group Medecins Sans Frontieres, which runs several clinics in Badghis, said three of its foreign staff and two Afghans were involved in an incident in the province, but had no details and could not confirm any deaths.
The official, who spoke on condition he not be named, declined to give the foreigners' nationalities.
Naibzada said he didn't know where the foreigners were from and that it was unclear if they were the victims of anti-government militants or robbers. "It's too early to say who's behind this," he said by telephone from a car rushing him to the scene.
But Mullah Abdul Hakim Latifi, a purported spokesman for the Taliban, called AP and said it carried out the attack at 4 p.m. and gave the location where it took place. He threatened more attacks would follow.
"The Taliban was responsible for this attack," Latifi told The AP. "Those international aid workers were working for the policy of America. There will be more of these attacks in the future."
Rebels of the former ruling Taliban regime and al Qaeda, active in the south and east of the country, have targeted aid workers over the past year in an apparent attempt to undermine efforts by the U.S.-backed government to rebuild the war-battered country.
But security is also poor in areas of the north, because of the continuing influence of rival warlords. Armed robberies are also regularly reported in the region.
Attacks by suspected Taliban militants are rarely reported in the north.
The violence has cast a long shadow over plans for general elections in September, seen as key to rehabilitating a nation scarred by a quarter-century of war.
In May, gunmen killed three U.N. election workers, including two British security consultants, in eastern Nuristan province. The Taliban claimed responsibility.
Also in May, two foreigners, one carrying a Swiss passport, were found dead in Kabul after being hit over the head with stones or bricks. Their identities and the motive for the killings remain unclear.
In November last year, two gunmen killed Bettina Goislard, a 29-year old worker for the United Nations High Commission for Refugees, in the eastern city of Ghazni, the only foreign U.N. staff member slain since the fall of the Taliban in late 2001.
On Saturday, four American special forces perished about 20 miles east of Qalat, the capital of southeastern Zabul province, when a bomb exploded under their Humvee.
The toll was one of the worst for a single attack on the U.S.-led coalition force, which currently numbers a record 20,000, since it entered Afghanistan to topple the Taliban for harboring al Qaeda leader Osama bin Laden in late 2001.
Overnight Saturday, Taliban guerrillas riding a fleet of vehicles shot up a government office in southern Afghanistan overnight, killing four Afghan soldiers and losing one of their own.
The suspected Taliban swept into Musa Qala, a market town 150 miles southwest of the capital, Kabul, late Saturday, opening fire on the government office with assault rifles and heavy machine-guns, mayor Mullah Amir Aghunzada told The Associated Press.
Four of the 30 soldiers defending the compound were killed and eight others wounded, Aghunzada said. One Taliban fighter was also killed and four captured, three of them wounded.
At least 89 American service personnel have died in and around Afghanistan since the start of the U.S. war on terrorism following the Sept. 11 attacks, including 55 killed in action.
But the vast majority of the 360 people killed in violence across Afghanistan this year are Afghan soldiers and civilians.
In a press conference on Tuesday, President Bush said, "the reports from Afghanistan, at least the ones I get, are very encouraging."
"You know, we've got people who have been there last year and have been back this year report a different attitude. And they report people have got a sparkle in their eye," the president said.
The United Nations and Afghan officials have so far registered some 2.7 million of the estimated 10 million Afghans eligible to vote. But poor security means they have been unable to enter rural areas in much of the south and east.
The U.S. military has insisted the vote can go ahead and announced Saturday that thousands of U.S.-trained troops from the new Afghan National Army will fan out across the country to help 20,000 German-trained police provide security.
NATO-led peacekeepers are also expected to expand north from Kabul in time for the vote. The 6,400-strong force took delivery Sunday of three Turkish Black Hawk helicopters to ease its shortage of equipment.