Dense fog is hampering the efforts of rescuers searching for the wreckage of an Afghan passenger jet that crashed with 44 people on board, officials say.

At least 70 rescue workers are searching on the ground but dense fog has continued to cover the area of the crash near the 12,700ft-high Salang Pass, a major route through the Hindu Kush mountains that connects Kabul, the capital, to the north.

The Russian-made AN-24 aircraft, operated by Pamir Airways, a private Afghan airline, was flying from the northern city of Kunduz when it crashed about 100km from Kabul on Monday.

Nato dispatched a fixed-wing aircraft to the last known position of the jet at the request of the Afghan government, but a spokesman said the aircraft had to turn back because of bad weather.

The Afghan defence ministry also ordered the nation's air force to be on standby.

Offcials said there was no immediate word on the passengers' fate, who included six foreigners.

The British embassy in Kabul confirmed that three UK citizens were on the aircraft, but did not identify them.

One American also was on board, a state department official in Washington said on condition of anonymity pending notification of family.

The nationalities of the two other foreigners were not immediately available.

'Conditions bad'

Al Jazeera's Hoda Abdel-Hamid, reporting from Kabul, said: "The conditions are bad. It's so foggy there is no rescue going on.

Referring to the Nato force in Afghanistan - the International Security Assistance Force - she said: "Two Isaf helicopters are en route to the area. Other Isaf helicopters are also on standby ... to assist in any rescue effort."

Colonel Nabiullah, who is in charge of the southern portion of the Salang Pass, said: "The only way they can search is on foot. The helicopters can't get in."

Afghanistan's acting transport minister said the cause of the accident was not yet known.
  
"We have to recover the black box to determine the cause," Mohammadullah Bataz said.
  
"We cannot rule out any of the usual causes behind crashes; which could be bad weather, technical error, thunder and lightning or even terrorist attacks."

Other officials blamed the weather for the crash.

Pamir Airways is one of three major private airlines that operate mostly domestic routes across Afghanistan.

It has daily flights to major Afghan cities and also operates flights to Dubai and Saudi Arabia for the Haj pilgrimage.

According to its website, it uses Antonov An-24 type aircraft on all its Kunduz-to-Kabul flights.