US Defense Secretary Leon Panetta on Monday said insider attacks on NATO forces in Afghanistan were a "last gasp" tactic by Taliban insurgents who had been unable to make up lost territory.

His comments came after a weekend in which six NATO troops were killed in apparent green-on-blue attacks, and appear to contradict commanders on the ground, who say most of the assaults are the result of cultural conflicts.

"This is an approach that the Taliban is resorting to, similar to the use of IEDs (improvised explosive devices)," Panetta said during a visit to Tokyo.

He said "frankly it is a kind of last-gasp effort to be able to not only target our forces but to try to create chaos."

The Islamist insurgency has to resort to such tactics as "they've been unable to regain any of the territory that they have lost."

The US military was taking the threat seriously and looking at further steps to protect troops in Afghanistan, he said during a joint press conference with his Japanese opposite number.

However, Panetta insisted the turncoat attacks would not force a change in war strategy, which calls for advising Afghan forces until they take over security for the whole country by the end of 2014 -- paving the way for the withdrawal of NATO combat troops.

"We will do all we can to minimize those risks, but we will not lose sight of the fundamental mission here," he said.

"We're going to stick to that mission."

The deaths at the weekend of four US troops and two Britons took to 51 the number of Western soldiers killed by Afghan colleagues in 36 incidents so far this year.

NATO is gradually withdrawing its 112,600 remaining troops. The Pentagon said last week that there are currently 77,000 US troops in Afghanistan.

Panetta was in Tokyo on the first leg of a tour that will also take him to Beijing and to Auckland.

During his visit he addressed around 350 members of the US armed forces stationed at Yokota near the Japanese capital, a fraction of the 47,000 who live and work in the country.