"The last time that we have been told that the aircraft was seen on radar was about 3.1 miles east of Kabul," Transport Minister Enayatullah Qasemi said at a news conference. "Since this morning we have begun a search and rescue operation in the area."
Most of the 96 passengers are believed to be Afghans, but the U.S. Embassy said several Americans are also believed to be on board.
The Kam Air Boeing 737 took off Thursday afternoon from the western Afghan city of Herat bound for the capital but was unable to land because of a snowstorm.
Feda Mohammed Fedayi, the airline's deputy director, says the plane was diverted to an airport in Pakistan, possibly the border city of Peshawar, but so far the airline has received no word on whether it landed safely.
Pakistani aviation officials say the plane never entered their airspace.
"We have information that a plane went missing on the Herat-Kabul route but no plane has entered Pakistani territory and no plane has made contact with any air control tower," said Abid Rao, deputy chief of Civil Aviation for Pakistan. "We have checked all of our stations."
The manager of Peshawar airport, Syed Zahoor Ali Shah, also said his staff had no word of the plane.
"It is not possible that a plane would land in Peshawar and we would have no information," he said.
Fedayi, who went to Kabul airport along with worried government officials on Friday morning, said Kam Air had had no contact with the plane, which was carrying 96 passengers and eight crew members, since about 3 p.m. on Thursday.
He said the plane had radioed the main U.S. military base at Bagram, north of Kabul, during the flight to check on the weather, but didn't ask the American controllers for permission to land.
The company's flights are popular with Afghans wealthy enough to avoid long journeys over bumpy roads. Aid workers also use them, though most opt for special flights operated by the United Nations or foreign contractors.
Beth Lee, a spokeswoman for the U.S. Embassy in Kabul, said "there are Americans believed to be on the plane," and that officials were trying to establish their identities and how many.
Fedayi said the crew was from Kyrgyzstan, and that the pilot held a Canadian passport, but had no further details of who was on board.
Officials in the eastern Afghan city of Jalalabad, which lies on the route toward Peshawar, said the plane hadn't landed there.
Maj. Mark McCann, a spokesman for the U.S. military, was unable to confirm whether the plane had crashed, but said American and NATO officials are meeting with government and airline representatives to discuss its fate and whether to launch a search operation.
Afghan government officials declined to comment ahead of a noon news conference with the transport minister.
Like much of Afghanistan, the border is dominated by high mountains, raising the hazards for planes flying in bad weather and any rescue attempt. The area is so remote that officials suspect militants including Osama bin Laden have hidden there since the fall of Afghanistan's former Taliban government in 2001.
The last major plane crash in Afghanistan was on Nov. 27 last year, when a transport plane under contract to the U.S. military crashed in central Bamiyan province, killing three American soldiers and three American civilian crew.
The most recent commercial crash was on March 19, 1998, when an Ariana Airlines Boeing 727 slammed into a mountain near Kabul, killing all 45 passengers and crew.
Kam Air was the first private airline established in Afghanistan after the fall of the Taliban in 2001 and made its maiden flight on the Kabul-Herat route in November 2003. The airline operates a fleet of leased Boeing and Antonov aircraft on domestic Afghan routes as well as to Dubai.
Kabul has been hit by heavy snow and all flights out of the capital were canceled on Thursday.
By Stephen Graham