US Secretary of Defence Leon Panetta arrived in Kabul on Tuesday to reaffirm America's commitment to Afghanistan as Washington starts to withdraw combat troops and hand over security to Afghan forces.

The Pentagon chief, who says violence is falling in the country where the United States has been at war with the Taliban for 10 years, is slated to meet Afghan President Hamid Karzai and Defence Secretary Abdul Rahim Wardak.

The trip comes after America's fragile alliance with Pakistan plunged into further crisis when NATO air strikes killed 24 Pakistani soldiers near the Afghan border on November 26.

"I think 2011 will mark a turning point with regards with the efforts to Afghanistan," Panetta told journalists travelling to Kabul, his second trip to the country since taking up his post in July.

"Troops have been able to reduce the levels of violence there. They are successful in securing some of the key areas in Afghanistan. There's greater success in the Afghan military and police."

On arrival in Kabul Panetta met with General John Allen, the US commander in Afghanistan, who said troops would increasingly turn to a more advisory role as security is handed over to Afghan forces to take full control by 2014.

"We are going to probably see the introduction, timely yet to be determined, of some advisory forces to support ANSF (Afghan National Security Forces) formations from inside," Allen told reporters after the meeting.

"The crossover point when we become largely an advisory and assisting and educating force versus a force that is engaged at any given moment in counterinsurgency, remains to be determined."

The United States has announced the withdrawal of 10,000 of its troops by the end of the year, while 23,000 others will leave the country by the end of September 2012 after the summer "fighting season".

The number corresponds to the 33,000 "surge" troops ordered in by President Barack Obama in late 2009 in a bid to reverse the Taliban insurgency, defeat Al-Qaeda and quicken an end to the war.

Allen said this would leave 68,000 US troops but there would also be another 38,000 non-US ISAF troops and 352,000 ANSF forces.

"For us the number is less important than the strategy," he said.

Panetta earlier welcomed the "significant achievement" of a second phase of transition from NATO to Afghan security control, which was launched this month in conjunction with the planned exit of all NATO combat troops in 2014.

In the second wave, Afghan forces will take charge of six provinces, seven provincial capitals and more than 40 districts, including three in the southern province of Helmand, one of most deadly in the conflict.

Panetta said it was important to reach out to Pakistan, where US officials want Islamabad to dismantle Afghan militant havens.

"It's been said a number of times that ultimately we can't win the war in Afghanistan without being able to win in our relationship with Pakistan as well," said Panetta.

Pakistan shut the US supply line into landlocked Afghanistan on November 26, saying that the blockade could last weeks longer, and forced Americans to leave an air base widely reported to have been a hub for CIA drones.

US officials have accused Pakistan of supporting the Afghan Taliban and its Haqqani faction, whose leaders are based on Pakistani soil.

"They have provided cooperation in areas that have been important to us and at the same time we've had some serious difficulties with regards to the operations that involve groups in the FATA (Pakistan's tribal belt) and groups along the border," Panetta said.

"In the end Pakistan and the US do share the same concerns with regards to terrorism."

Allen, who spoke with Pakistan army chief General Ashfaq Kayani on Monday said they had a "cordial" discussion on restoring the border.

Pakistan called the November 26 attack deliberate. US commanders deny it was intentional. US investigators are due to submit a report on December 23.

Panetta, who made a brief stop in Djibouti before arriving in Kabul, will next travel to Iraq, Turkey and Libya.