The US marine corps has been broadcasting and handing out leaflets about their intentions to launch a major assault in Helmand province, Chater said.
"They [the Taliban] have had many days to prepare for this - it's the same sort of situation I saw in Iraq when the assault on Fallujah came about; everyone knew that was going to happen at the time," he said.
In some preliminary operations, about 32 Taliban and three soldiers have already been killed in Helmand province, the provincial government said on Thursday.
The operation took place in Nad Ali district, west of the provincial capital Lashkar Gar. Daud Ahmadi, a provincial government spokesman, confirmed the toll.
The Afghan defence ministry says the anti-Taliban push will be led by Afghan security forces as part of plans to hand over military and police responsibility to the Western-backed government.
"It will be a joint Afghan army, police, US Marines and Isaf forces operation led by Afghans," Mohammad Zahir Azimi, a defence spokesman, said.
The operation will feature the largest contingent of the Afghan National Army yet deployed on such an attack.
"They're due to take the lead in these sort of attacks by the end of this year, so its their performance that's going to be critical in this assault," Chater said.
The Taliban has said their forces will fight back to "the last drop of their blood".
Eric Tremblay, a Nato spokesman, says the aim of the assault in Helmand is to "clear, hold and build, and deliver alternatives for all the Afghans".
The United States and Nato have about 113,000 troops fighting in Afghanistan, with another 40,000 being deployed in the coming months.