Simon Bowden: "Up to 200,000 troops could be involved"

The exercises had been postponed while Iran awaited the outcome of United Nations mediation over differences with Afghanistan's Taleban leaders.

Iranian officials say military units have launched nocturnal assaults on mock enemy targets in the Zabol region in the south-east of the country.

The Iranian news agency said the operation was conducted under the light of flares and with the support of artillery, mortar and missile units.

More than 200,000 troops are reported to be involved, along with jet fighters and hundreds of tanks, although some sources put the figures much lower.

The operation is reported to be taking place over more than 50,000 square kilometres.

The diplomats' bodies were returned to Iran for burial

Iran's military deployment follows the killing of nine Iranian diplomats by Taleban troops as they seized the north Afghan city of Mazar-i-Sharif from Iranian-supported opposition forces in August.

Iran reacted by sending thousands of troops to the border and starting low level manoeuvres.

The BBC Kabul correspondent, William Reeve, says the UN special envoy, Lakhdar Brahimi succeeded in averting open war between the two countries.

The Taleban have also stepped up the military border presence

The Taleban have freed most of the Iranian prisoners they held in Afghanistan and agreed to meet the Iranian authorities face-to-face.

Iran still insists that the killers of its citizens be punished and that the Taleban abide by UN resolutions calling for broader government in Afghanistan.