The US Army has launched a major operation in support of a planned US-Afghan attack on the largest Taliban-controlled town in southern Afghanistan.

About 400 US troops from the 5th Stryker Brigade were joined by 250 Afghan soldiers and their 30 Canadian trainers to move into positions north east of Taliban-controlled Marjah.

There were no immediate reports of casualties, but large plumes of smoke could be seen in the area and reporters travelling with the US unit reported hearing the distant rattle of machine gun fire and detonations from rocket-propelled grenades.

US officials have not said when the main attack on Marjah will take place.