An Afghan police commander and 13 junior officers have joined the Taliban in the western Afghan province of Farah, in what correspondents say could be the biggest defection by police.

They say the commander, named as Mirwais, was in charge of a 20-man checkpoint when he defected on Sunday.

The men are said to have taken heavy weaponry, radios and police vehicles including US-made armoured Humvees.

Farah is one of the most insecure areas in the relatively peaceful west.

The commander was based in Shewan village in the district of Bala Bulak, which was until recently considered a Taliban stronghold.

The insurgents were driven out of the area following a series of operations carried out by Afghan security forces. But local officials say insurgents have regrouped in the area recently.

Rarely reported

Police and intelligence officials deployed in the province said the commander poisoned seven policemen in his charge who had refused to defect along with him.

Analysis Government officials, both in Kabul and the provinces, are shocked and confused. How could a senior police commander for a strategic district have defected? It is a huge embarrassment for the Afghan government - which also explains its silence. There has been no press release, no statement. The incident took place late on Sunday night but full details emerged only on Tuesday. But this is a clear case of a major intelligence failure and a further propaganda boost for the insurgents. The reality is that commanders like Mirwais are key to the security and stability of remote villages and areas, where local deal-making and contacts are crucial to compensate for a weak Afghan government.

"Mirwais and his policemen had joined the force nearly two-and-a-half years ago. Mirwais had fought the insurgents in this area for quite some time," an Afghan intelligence official in the region told the BBC's Bilal Sarwary in Kabul.

"Long before he defected, he must have been passing intelligence and crucial information to the insurgents," the official said.

Officials said that the equipment taken by the defectors - rocket-propelled grenades, heavy machine-guns, radios and police vehicles, including two Humvees - will be a major boost to the Taliban in the area.

Humvees are prized trophies among Taliban commanders, both for their symbolic value and practical ability to travel over rough ground with armoured protection, correspondents say.

Our correspondent says this is believed to be the biggest police defection to the insurgents. Over the past few years similar incidents have taken place on a much smaller scale in the southern provinces of Kandahar, Helmand, Zabul, Uruzgan and in Ghor, Farah, Badghis and Herat in the west.

Such incidents have rarely been reported in the Afghan media, our correspondent says.

International forces have recently had increasing success in persuading Taliban fighters to come over to the government side.

Once relatively peaceful, Farah has seen increasing levels of violence. The Taliban and other insurgent groups are active in many districts, particularly those close to the provincial capital, which has witnessed recent attacks.

The province has a strategic position, bordering Iran, and the key Kandahar-Herat highway passes through Farah.