The attack occurred in the southern city of Kandahar, the birthplace of the Taliban movement that has waged an increasingly fierce insurgency against targets including civilians since being ousted in 2001.

"Four Nepalese and an Afghan who were working for the UN security services were killed in a very powerful remote-controlled explosion," Kandahar province police chief Esmatullah Alizai told AFP.

The bomb almost completely destroyed the armoured UN vehicle and its smoking wreckage lay across a road near the centre of the city. Police were trying to remove two bodies trapped inside, an AFP reporter witnessed.

"We're investigating what type of explosive was used. It was a very powerful blast," Afghan interior ministry spokesman Zemarai Basharay said while confirming five people had died in the blast.

UN spokesman Aleem Siddique said there was a "roadside bomb explosion against a UNOPS (UN Office for Project Services) convoy which was travelling to its base in Kandahar" but had no details of casualties.

There was no immediate claim of responsibility for the attack, but previous blasts have been blamed on the fundamentalist Islamic Taliban militia.

Meanwhile, four Afghan employees of a US-owned private security firm were killed by a motorcycle-borne suicide bomber on Sunday at Kandahar airfield, a base for the Nato-led International Security Assistance Force.

Human Rights Watch said in a report published Monday that civilian deaths from "war crimes" and other attacks by Taliban-led insurgents have soared in the past 15 months.

At least 669 Afghan civilians had died since the start of 2006, the report said.

A would-be suicide bomber wearing an explosives-filled vest was detained on Monday night in the eastern city of Khost, where eight policemen were killed in a powerful suicide blast on Saturday, Khost police chief Mohammad Ayoob said.