US Defense Secretary Leon Panetta told troops in Afghanistan on Thursday that the decade-long war was at 'a turning point', as Kabul reacted with fury to a NATO air strike that killed up to 18 civilians.

Panetta arrived in the Afghan capital on his second visit in less than three months as President Hamid Karzai branded Wednesday's air strike on a home in Logar province 'unacceptable' and cut short a visit to China.

The United States, which leads 130,000 NATO troops fighting a Taliban insurgency, is planning to withdraw the bulk of combat forces from Afghanistan by the end of 2014 and hand responsibility for security to the Afghans.

Panetta noted a recent 'uptick' in violence and said a double suicide attack on Wednesday outside the largest NATO base in the south that killed 23 people was 'much more organised than we've seen before'.

But the US defence chief sought to reassure soldiers that their sacrifices had not been in vain and Afghans that NATO's drawdown did not mean they would be abandoned.

Noting the end of the US war in Iraq, Panetta told soldiers gathered at the heavily fortified Kabul airport that 'hopefully we'll be able to accomplish the mission in Afghanistan as well'.

US commanders have 'put a very good plan in place' and Afghans worried about the withdrawal should know 'that we're not going any place', he said in a reference to plans to keep a residual force in Afghanistan.

The post-2014 role, the size of which is yet to be determined, would include fighting 'terrorism' and training and advising, he said.

"We've lost a lot people in battle. We've got to make damn sure they didn't die in vain."

Panetta said the Taliban had been 'weakened', but noted a recent 'uptick' in violence, saying: "The reason for that is we've taken the battle to them."

A day before Panetta's arrival, Afghan officials said about 40 civilians were killed in the twin suicide bombing near Kandahar Air Field and in the NATO air strike south of Kabul.

"I know this is still not going to be an easy fight," Panetta said, vowing to confront the Haqqani network, a Taliban-linked group blamed by the United States for some of the worst violence in the country.

"We're also dealing with a safe haven," he said, referring to Haqqani bases in neighbouring Pakistan.

"We are going to make very clear that we are going to take them on," he said.

US relations with Pakistan are in free fall, in part over American drone strikes that target Islamist militants in Pakistan's northwest and US distrust that Pakistan is playing a double game in supporting Afghan insurgents.

Panetta told reporters that he wanted to hear an assessment from commanders about a recent rise in insurgent attacks and plans for troop withdrawals.

He held talks with the commander of NATO-led forces, US General John Allen, and the US ambassador to Kabul, Ryan Crocker, who has announced after less than a year on the job that he is leaving his position.

Panetta is also due to meet Afghan Defence Minister Abdul Rahim Wardak with the US-led International Security Assistance Force investigating reports that civilians were killed in an air raid on a home in Logar province.

"Attacks by NATO that cause life and property losses to civilians under no circumstances could be justified and are not acceptable," Karzai said of the attack in a statement that announced he was cutting short a visit to China.

Local police said up to 18 civilians, including women and children, were killed in the coalition air raid.

Civilian casualties caused by US and NATO air strikes have been a frequent source of tension between Karzai and the United States.

Panetta's trip to Kabul comes at the end of a nine-day tour through Asia, including stops in Singapore, Vietnam and India, but significantly not Pakistan.

During the trip, he portrayed the war as winding down while the United States shifts its focus to the Asia-Pacific region.