Several men have been killed in a gun battle in eastern Afghanistan, after a police vehicle escorting a United Nations convoy came under fire.

Gunmen attacked a convoy from the UN refugee agency, the UNHCR, as it was travelling through Nangarhar province, about 40 kilometres (25 miles) west of Jalalabad.

Two policemen were killed when the first shots were fired. Guards at a nearby checkpoint then opened fire on their attackers. Another four men are believed to have died in the ensuing violence.

There were no casualties among the UN staff, all of whom have now returned safely to their office in Jalalabad.

One of the alleged attackers was later arrested.

The BBC correspondent in Kabul, Kylie Morris, says the district where the attack happened is notorious for banditry and the growing of opium poppies.

The Afghan security commander for Jalalabad says he thinks the attackers were opposed to the work of the international community in reconstructing Afghanistan.

But a UN source has told the BBC he thinks it is more likely the attack resulted from tensions connected to the current ban on opium production and disputes between local commanders.