MARJAH, Afghanistan — Marines moving by land from the north linked up today with U.S. units that have faced nearly constant Taliban attack in the four days since they were dropped by helicopter into this insurgent stronghold in southern Afghanistan.

Also today, U.S. artillery fired nonlethal smoke rounds to disperse Taliban fighters in Marjah — the first time cannons have been used in the fight to drive the militants from their logistical and opium poppy-smuggling base. Commanders refused a Marine request to fire deadly high-explosive rounds because the unit on the ground could not be sure civilians weren't at risk.

The linkup between the two Marine rifle companies and their Afghan army partners will enable the U.S. to expand its control in Marjah, situated in Helmand province 380 miles southwest of Kabul.

Lima Company of the 3rd Battalion, 6th Marines moved through fields of hidden bombs and bobby traps and braved heavy sniper fire to join up with the same battalion's Kilo Company, which was airdropped into the town in the first hours of the operation Saturday.

Lt. Gordon Emmanuel, a platoon commander in Kilo Company, said the Marines landed without encountering Taliban fire but came under sustained attack as they fanned out from the landing zone.

"When it is daytime, there is nonstop contact until the sun goes down ... every day," Emmanuel said.

About 15,000 NATO and Afghan troops are taking part in the big offensive around Marjah, which has an estimated 80,000 inhabitants and was the largest southern town under Taliban control. NATO hopes to rush in aid and public services as soon as the town is secured to try to win the loyalty of the population.