The confrontation occurred early Sunday on the Tohono O'odham Indian Reservation, about 85 miles southwest of Tucson, in an area fenced only with barbed wire, said Dove Crawford, a spokeswoman for the Border Patrol.

The soldiers, outfitted in desert camouflage, pointed their rifles at the agent and shouted at him not to move, Crawford said. They lowered their weapons after about four minutes when the agent convinced them of who he was and where they were, she said. The soldiers then retreated into Mexico.

The Mexican government has sent soldiers north along sections of the border in efforts to tamp down drug-related violence.

State Department spokesman Gonzalo Gallegos in Washington said the encounter "stemmed from a momentary misunderstanding as to the exact location of the U.S.-Mexican border."

Portions of the Tohono O'odham reservation's 75-mile border with Mexico have been reinforced with vehicle barriers or other fencing to keep out illegal immigrants and drug smugglers. But other areas on the reservation and other stretches of border either have only barbed wire or no markers at all, making it hard to distinguish the boundary.

"There are places where there are no markers, at least not easily found," said Lloyd Easterling, a Border Patrol spokesman in Washington. "There's no line painted in the sand or anything like that."

He added: "We're working with the Mexican government to make sure that this doesn't happen again. This can't be happening."

Mexican Embassy spokesman Ricardo Alday in Washington said in a statement that both countries are investigating.

"It seems that there are dissimilar versions of what happened, and both countries are trying to clarify it together," Alday said.

"Law enforcement operations have led, from time to time, to innocent incursions by both U.S. and Mexican law enforcement personnel and military units into the territory of both nations, and in particular along the non-demarcated areas of our border," he said.

T.J. Bonner, president of the National Border Patrol Council, the union for Border Patrol agents, said the incursions have created a disturbing pattern.

Bonner said there have been at least a half-dozen situations in recent years in which Mexican soldiers have entered U.S. territory and shot at Border Patrol agents.

"It's a minor miracle that none of our agents have been killed or seriously injured," he said.

"It's inexcusable to not know where the border is" when military units have global positioning capabilities, Bonner said.

The instances in which the Border Patrol strays into Mexico are few and far between, Bonner said.

And "we have no incursions with Canada," he added. "Absolutely none."