A plane has crashed in southern Afghanistan, killing four military personnel, the Nato-led International Security Assistance Force (Isaf) says.

A spokesman said the cause of the crash was being investigated, but initial reports suggested there was no militant activity in the area at the time.

He did not give the nationalities of the victims or the crash's location.

However, the deputy governor of Zabul province said a foreign military aircraft came down there on Saturday.

Mohammad Jan Rasoulyar told the Associated Press that the scene of the crash in Shah Joy district had been surrounded by troops.

Last month, five US troops were killed when a helicopter crashed during a rainstorm in the southern province of Kandahar.

'Spring offensive'

In a separate development on Saturday, there have been reports of fierce fighting in the northern province of Faryab.

Details are still unclear, but insurgents are understood to have taken control of several villages in Qaisar district.

A spokesman for the provincial governor, Jawed Baidar, told the BBC that more than 60 insurgents had been killed. Women and children are also thought to have been among the casualties.

Faryab has generally had a reputation for being a relatively peaceful part of Afghanistan, despite a high-profile suicide bombing at a mosque in the provincial capital Maymana last October that left at least 41 people dead.

Earlier, the Taliban's leadership announced that the group's annual "spring offensive" would begin on Sunday.

"Every possible tactic will be utilised in order to detain or inflict heavy casualties on the foreign transgressors," a statement said.

The Taliban threatened to start a new campaign of suicide attacks on foreign military bases and diplomatic facilities, as well as infiltrate Afghan security forces and carry out 'insider attacks".

The Afghan defence ministry said the army was "ready to neutralise the offensive", adding that its soldiers had the trust and support of Afghans.