The United States is running out of patience with Pakistan over safe havens for insurgents who attack US troops across the border in Afghanistan, Defencee Secretary Leon Panetta warned on Thursday.

Panetta was speaking during a brief visit to Kabul overshadowed by Afghan fury over a NATO air strike that allegedly killed 18 civilians -- an issue that the Pentagon chief did not mention at a news conference.

Panetta left for the airport just hours after his arrival, as Afghan President Hamid Karzai pledged to cut short a trip to Beijing and head home over the deaths of around 40 civilians Wednesday in the air strike and a suicide bombing.

Panetta, in Kabul to assess the state of the war and plans to withdraw US combat troops by the end of 2014, lashed out at the Haqqani network and Pakistan over a recent increase in violence in Afghanistan.

The Haqqani group, a faction linked to the Taliban and Al-Qaeda that is believed to be based in Pakistan's lawless tribal district of North Waziristan, is blamed for some of the deadliest attacks in Afghanistan's 10-year war.

"It's an increasing concern that Haqqani safe havens still exist on the other side of the border. Pakistan has to take action (against) allowing terrorists in their country to attack our forces on the other side of the border," Panetta said.

"We are reaching the limits of our patience here," he added at the news conference with Afghan Defence Minister Abdul Rahim Wardak.

The Afghan and US governments have said they do not believe the war in Afghanistan can be won without safe havens in Pakistan being dismantled. Critics in Pakistan have accused them of deflecting blame for the increasingly deadly war.

The United States leads 130,000 NATO troops fighting the Taliban insurgency and 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 said that in talks with Pakistan, the United States had made "very clear, time and time again" the need to crack down on Haqqani militants.

But Karzai, who was attending a meeting of the Shanghai Cooperation Organisation in Beijing, issued a stinging rebuke of the West for the deaths of civilians in a bloody day on Wednesday.

"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 air strike Wednesday in Logar province, south of Kabul.

Karzai 'is deeply grieved' over the deaths of civilians in the NATO strike and in a Kandahar suicide bombing on the same day, and 'will shorten his trip to China and will very soon return to the country,' his office said.

NATO's US-led International Security Assistance Force (ISAF) said 'multiple insurgents' were killed in the air strike, which was ordered after troops came under fire during an operation against a Taliban commander.

But ISAF said it would launch an investigation after local police said 18 civilians, including women and children, were killed in the strike.

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

For the past five years the number of civilians killed in the war has risen steadily, reaching a record of 3,021 in 2011, with the vast majority caused by insurgents, the United Nations says.

In the Kandahar suicide attack on Wednesday, 23 civilians were killed when two Taliban bombers blew themselves up at a makeshift bazaar and truck stop near a major NATO base.

Panetta told US troops in a speech at the heavily fortified Kabul airport that the decade-long war was at 'a turning point'.

He 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.

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."