Two kidnapped foreign doctors and their three Afghan colleagues are being held in the remote province of Badakhshan by an armed criminal group, police say.
They are now being held in the Shahr-e-Bozorg area after being abducted while travelling on donkeys north of the provincial capital Faizabad.
Senior Afghan intelligence sources say that they have established contact with the kidnappers.
They are trying to establish what the kidnappers' demands are.
The area is remote, mountainous and deeply forested.
"We have established contact with [the kidnappers] both via telephone and tribal elders, and are waiting to see what they want," an intelligence official told the BBC.
The names of the aid workers, and the name of the organisation they work for, have not been officially released.
The five were working as part of a team providing medical assistance to remote and isolated communities in Yaftal district - about 90km (56 miles) north of Faizabad.
Badakhshan province has generally been seen as one of the safer parts of the country, but there have been more violent incidents and indications of increased Taliban activity in recent years, says the BBC's Andrew North in Kabul.
Badakhshan province borders Tajikistan, China and Pakistan.
Police in the area of the kidnappings say that while it is not a focus for insurgents, it is home to groups of mainly criminal gunmen.
They said that the aid workers had been visiting a health clinic in Yawan district, where the road had been destroyed by floods.
"They were travelling by donkey. Two foreign women and their Afghan translators were kidnapped and we have started a search operation," police spokesman Lal Mohammad Ahmadzai told the Reuters news agency.
Nato-led forces said they had no information on the kidnapping, while aid agencies operating in the area have so far declined to comment because of the sensitivity of the issue.
Correspondents say that the kidnapping of foreigners has become relatively common in parts of Afghanistan since the US-led invasion of the country in 2001.
Badakhshan has also been hit by growing violence.
Last month a prominent anti-Taliban commander in Badakhshan, Nazik Mir, was killed in a suspected Taliban suicide attack.
In August 2010, British doctor Karen Woo was killed in an ambush in the province along with six Americans, one German and two Afghan translators.