US Defense Secretary Leon Panetta has voiced frustration with Afghan President Hamid Karzai preferring to "criticize" American troops, rather than acknowledging the sacrifices they have made.

Panetta, who arrived in Peru late Friday to begin a Latin American tour, told reporters aboard the military plane taking him to Lima that Karzai should remember that more than 2,000 US troops have died in Afghanistan.

The angry riposte came after Karzai said on Thursday that the United States was failing to go after militants based in Pakistan, another charge that Panetta chose to hit back at.

Speaking at a press conference in Kabul, Karzai accused the United States of playing a "double game" by fighting a war against Afghan insurgents rather than their backers in Pakistan where, in Karzai's words, "terrorism is financed and manufactured."

The Afghan president also lamented what he described as NATO's refusal to supply Afghanistan with modern weapons necessary to fight its enemies.

But a visibly displeased US defense secretary suggested the Afghan president had focused on the wrong things.

"We have made progress in Afghanistan because there are men and women in uniform who are willing to fight and die for Afghanistan's sovereignty and their right to govern and secure themselves," Panetta said.

"We've lost over 2,000 US men and women, ISAF has lost forces there and the Afghans have lost a large number of their forces in battle.

"Those lives were lost fighting the right enemy, not the wrong enemy. And I think it would be helpful if the president, every once in a while, expressed his thanks for the sacrifices that have been made by those who have fought and died for Afghanistan rather than criticize."

The outburst was rare for Panetta and the remarks came as relations between the United States and Afghanistan are under strain in the wake of several deadly and high-profile attacks on US troops by their local comrades.

In Afghanistan, the United States has also seen its image tarnished among ordinary Afghans this year by the burning of Korans at a military base, the abuse of corpses and a massacre of civilians by a rogue American soldier.

An unprecedented number of Afghan security personnel have turned their weapons against their allies, killing at least 51 NATO soldiers this year.

Despite this, many Afghans, particularly in the cities, fear the departure of the Western troops in 2014 from a country where the government of Karzai is widely seen as corrupt and dependent on foreign support.

In Lima, Panetta will meet President Ollanta Humala and Defense Minister Pedro Cateriano, Pentagon press secretary George Little told reporters.

"This trip will reaffirm the department's commitment to strengthening partnership around the world, particularly in Latin America and Europe," said Little.

The United States has provided Peru with surveillance planes to disrupt drug traffickers and help tackle remnants of the Shining Path guerrilla group. Washington gave Lima $659 million in associated aid between 2006 and 2011.

A senior US defense official said Panetta would offer further help to Peru under the Ministry of Defense Advisers program, a scheme currently being used in Afghanistan.

"What MODA does is that it embeds technical experts within the MOD for one or two years and they can provide technical advice," the official said.

Panetta will be in Uruguay on Sunday, the Pentagon said, and he will take part in the 10th Conference of Defense Ministers of the Americas at the Punta del Este resort, before traveling to Brussels on Monday.