DAHANEH, Afghanistan — U.S. Marines battled Taliban fighters Wednesday for control of a strategic southern town in a new operation to cut militant supply lines and allow Afghan residents to vote in next week's presidential election.

Insurgents appeared to dig in for a fight, firing volleys of rocket-propelled grenades, mortar rounds and even missiles from the back of a truck at the Marines, who were surprised at the intense resistance. By sunset, Marines had made little progress into Dahaneh beyond the gains of the initial pre-dawn assault.

Fighting accelerated after sundown, and officers predicted a couple of days of intense combat before the town could be secured.

"Based on the violence with which they've been fighting back against us, I think it indicates the Taliban are trying to make a stand here," said Capt. Zachary Martin, commander of Golf Company, 2nd Battalion, 3rd Marines.

The operation, Eastern Resolve 2, was launched early Wednesday with 400 Marines and 100 Afghan troops, who leapfrogged over Taliban lines in helicopters to attack militant positions in mud-brick compounds at the edge of town.

It was the third major push by U.S. and British forces this summer into Taliban-controlled areas of Helmand province, center of Afghanistan's lucrative opium business and scene of some of the heaviest fighting of the Afghan war.

British troops have been responsible for Helmand the past three years, but never had enough forces to take and hold Dahaneh.

The Marines are part of the 21,000 additional forces President Barack Obama deployed to Afghanistan this year to stop the Taliban's violent momentum.

By their operations, U.S. and British troops hope to break the Taliban grip on the province, sever smuggling routes from Pakistan and protect the civilian population from Taliban reprisals so Afghans can vote here during the Aug. 20 election. The Taliban have called for a boycott of the ballot and threatened to ruin the election.

During the first day of fighting, Marines said they killed between seven and 10 militants and seized about 66 pounds of opium, which the Taliban use to finance their insurgency. The U.S. military said an American was killed by a bomb Wednesday in southern Afghanistan, but the statement did not say whether the blast was part of the fighting around Dahaneh.

At the Pentagon, spokesman Bryan Whitman said the operation was "going as planned."