The kidnapping has raised fears among Kabul's 2000-strong foreign community that they might fall victim to Iraq-style kidnappings, but investigators suspected the perpetrators were criminals rather than political fighters.
"There is a perception that a criminal group could be behind this although nobody has claimed responsibility," said President Hamid Karzai's spokesman, Jawed Ludin said on Tuesday.
Meanwhile, a spokesman for the Taliban has denied involvement.
"Our mujahidin have not kidnapped the woman. We don't indulge in such things," Abdul Latif Hakimi said by telephone. "We don't know who's done it."
The Pakistan-based Afghan Islamic Press (AIP) news agency, citing unidentified sources, said the woman had been kidnapped by robbers seeking the release of three detained comrades. They had set a deadline of 1pm, AIP said.
Security officials said they had no knowledge of a deadline.
"Our mujahideen have not kidnapped the woman. We don't indulge in such things"
Abdul Latif Hakimi,
The kidnapping followed warnings from security agencies that foreigners might be targeted in response to the arrests of suspects in the kidnappings of three UN election workers last year.
The woman, 32-year-old Clementina Cantoni, worked for US-based relief agency Care International, Paul Barker, the agency's director in Afghanistan, said.
Four men forced Cantoni into a white Toyota sedan in Kabul's Shahr-e-Naw district on Monday morning, General Mahboubullah Amiri, a senior official in the Afghan Interior Ministry said.
"Four men carrying Kalashnikovs bashed in the window of her car and took her away. They told the driver not to move or he would be shot," Barker said.
Marco Formigoni, a family friend, spoke to reporters outside the Cantoni family home in an upscale Milan neighbourhood, relaying the family's hope "that this affair ends quickly and well."
It was the second kidnapping of a Care worker in recent months. Margaret Hassan, the British director of Care International in Iraq, was kidnapped in Baghdad in October and believed killed, although no body was recovered.
In Kabul, security forces immediately sealed off all main roads leading out of Kabul, said Jamil Khan, head of the criminal investigation department for the city's police.
Police say they are trying hard
to trace the aid worker
Officers stopped and searched cars in the city centre, checking trunks and under seats.
"Police are trying very hard to produce some good news," Khan said.
Cantoni has lived in Afghanistan since 2002, said Care, one of the largest and most established international aid groups in the country. The organisation issued a brief statement calling for her release.
In Rome, the Italian Foreign Ministry said a crisis unit that had handled past abductions of Italians abroad was working on the case, and that Foreign Minister Gianfranco Fini was following the situation.
The abduction follows a string of warnings to the roughly 3000 foreigners living in Kabul that they could be targeted in attacks.