WASHINGTON — The first major operation launched with the additional troops ordered to Afghanistan by President Barack Obama is devised to clear Taliban havens across a strategic southern province — and then, in a marked departure from past practice, to leave clusters of Marines in small bases close to the villagers whom they were sent to guard and to aid, according to senior military officers.

Despite the troops' substantial numbers and firepower, the strategy is not without risks. Indeed, on Thursday, the first U.S. Marine was killed in the operation.

Although U.S. and allied forces have previously swept through the province, Helmand, killing or capturing as many guerrillas as they could, often with airstrikes, the military has never before had enough ground troops to hold onto large areas that were cleared of insurgent fighters in combat operations.

This time, a force of about 4,000 Marines, which arrived just weeks ago, was joined by 650 Afghan soldiers in the sweep into Helmand province, a hotbed of Taliban activity and the largest producer of the opium poppies that are a major source of money for the insurgency.

"Essentially what they are trying to do is create and sustain a productive presence in Helmand province, including both combat power and civil-engagement capabilities," a senior military officer said.

"This is not simply to remove Taliban influence, but to replace that influence with security operations and reconstruction," the officer said. "It is not simply about killing the enemy, but about protecting the population and improving their lives, which will help prohibit the return of insurgent elements."

Commanders promised that the Marines would remain in significant numbers, setting up many small operating and logistics bases among the population. That tactic was central to the success of the so-called troop surge in Iraq.

Marines pushed into three significant towns along a 75-mile stretch of the Helmand River south of Lashkar Gah on Thursday, encountering sporadic resistance that grew throughout the afternoon, a Marine spokesman said.

The Marines now have a presence in Nawa and Garmsir in central Helmand province, and in Khan Neshin in southern Helmand, said a spokesman for Marine forces in the province, Capt. Bill Pelletier. He described the contact with Taliban fighters as "mostly hit and run." Some Marines were treated for heat exhaustion because temperatures reached 110 degrees, he said.

Obama told the Associated Press in an interview Thursday he wants to help ensure that Afghans "are benefiting from development and improved agricultural systems and education systems and health care systems."