The U.S. Marines and Afghan army plan a massive assault on Taliban fighters in Marja, the last community under Taliban control in a sprawling, lawless region once dominated by the insurgency, a top Marine said Wednesday.

“We are going to gain control,” Col. George “Slam” Amland told reporters. “We are going to alter the ecosystem considerably.”

Amland, deputy commander of Marine forces in southern Afghanistan, would not discuss the timing of the assault or how many thousands of troops would be involved.

Amland said the assault would involve Marine units that are part of a troop buildup authorized by President Obama in December. The assault will also show how the Afghan army is growing in numbers and competency, he predicted.

“This is a big leap for the government of Afghanistan,” he said.

Marine and NATO leaders want Helmand province to be a showpiece of the “clear, hold, build and transition” counterinsurgency strategy, in which Taliban fighters are forced out of a region and then a “civilian surge” begins to rebuild war-ravaged communities and bolster confidence of Afghan villagers in their provincial and national governments.

Where once the Taliban controlled nearly all communities of the Helmand River valley, Amland said, by summer there will be no place for Taliban to hide except in mountainous regions with sparse populations.

While the military part of the operation is the most dramatic, the actions of U.S. civilian employees, including from the U.S. Agency for International Development and Agriculture Department, will be even more significant, he said. The Afghan government is ready to install local officials to begin reopening schools and clinics and polling residents about what they want their government to do.

The goal, Amland said, is to spread to Marja the “kinds of success” seen in other communities once the Taliban were ousted. In the Nawa district of the province, for example, the marketplace reopened, irrigation canal clearing projects started, and a local community council was established once the Taliban fled.

Starting in June, battalions of Marines swept into Helmand, pushing Taliban fighters away from other communities. Hundreds, maybe thousands, fled to Marja, which the Marines opted not to enter. Last year the Afghan army’s presence was limited and its effectiveness doubtful.

Marja, with a population estimated at 85,000, has been a “sore” hampering U.S. and Afghan efforts in the province, Amland said. From Marja, Taliban have built roadside bombs, plotted assassinations and controlled the illicit poppy crop, which provides 60% of the world’s heroin and funnels profits into the Taliban insurgency.

By ousting Taliban from control of Marja, U.S. , NATO and Afghan officials hope to persuade rank-and-file, non-jihadist fighters -- what Amland called “lunch-bucket $5 a-day Taliban” -- to quit fighting and decide to see if the Afghan government can provide a better life for its citizens.

In the interim, the U.S. plans a “cash for work” plan to give jobs to the unemployed of Helmand province, including young men who may have joined the Taliban as an economic necessity.

While the assault well be sizable in scale, Amland said, it is the kind of mission for which Marines continuously train.

“It’s nothing we haven’t done before; it’s nothing we won’t do again in the future,” he said.

tony.perry@latimes.com