The U.S decided to keep the additional forces in the country temporarily because planned troop commitments by U.S allies for a NATO train-and-assist mission starting in January have been slow to materialize. NATO countries were supposed to contribute about 2,200 troops to the residual force, for a total of about 12,000.

Hagel, who arrived in Kabul on a previously unannounced trip one day after Obama announced he would nominate Ashton Carter to succeed him, was making his fourth and final trip as Defense secretary to Afghanistan. He was brimming with optimism about the prospects for stability, saying he believes the Afghans will stifle the recent surge in Taliban attacks in Kabul.

The Taliban, Islamists who ruled the country from 1996 until ousted by U.S.-led forces in 2001, regrouped substantially after the U.S shifted its military focus to Iraq in 2003. They are now trying to destabilize Ghani's new government.

Hagel said they have failed in that effort so far and are not likely to succeed.

"I have confidence in the Afghan security forces that they will continue to meet these challenges," he said.

However, he said it's not surprising the Taliban are able to mount significant attacks in the capital.

"The Taliban are going to continue to have pockets of resurgence, and it's predictable that they would do everything they possibly could to disrupt" the new government.

In September, the United States and Afghanistan signed a long-delayed security agreement to allow 9,800 troops to remain past the end of this year. U.S and Afghan officials had agreed on terms of the accord over a year earlier, but then-Afghan president Hamid Karzai refused to sign, complaining that U.S airstrikes had killed Afghan civilians and that the U.S had made overtures to the Taliban.

Last month, President Obama approved a plan to let U.S troops protect themselves and allies in Afghanistan. The plan allows airstrikes to aid U.S., allied and Afghan troops, but was not intended to allow offensive operations against the Taliban.

The changes reflect a more cooperative role with the new government in Kabul; Ghani enjoys a stronger relationship with the U.S than Karzai did.

The United States had a peak of about 100,000 troops in Afghanistan in 2010 and 2011, according to the Pentagon. The U.S combat mission officially ends Dec. 31, and U.S troops are to be removed entirely by 2017.

U.S Secretary of Defense Chuck Hagel is greeted after arriving Dec. 6 in Kabul.(Photo: Mark Wilson, AP)

  BERLIN, Dec 5 (Reuters) - Germany's Chancellor Angela Merkel pledged on Friday to support Afghan President Ashraf Ghani and his new unity government after NATO combat troops withdraw from the country at the end of more than a decade of fighting Islamist insurgents.

Afghanistan's 350,000-strong military and police forces have taken over security in most of the country, facing off against the Taliban in their first real test since the militant Islamists were ousted from power in 2001 by U.S.-led forces.

The United States and its allies intend to keep about 12,000 troops in Afghanistan for counter-terrorism and training of local personnel after combat forces formally withdraw this year.

Merkel told a joint news conference with Ghani in Berlin that Germany would also support Afghanistan in the longer term by training security forces and providing development aid.

"Germany has a responsibility, especially for security in northern Afghanistan," Merkel said, adding that bilateral economic ties were also becoming more important.

The German parliament voted earlier on Friday to keep 850 soldiers in Afghanistan in 2015 to train local forces, making Germany one of the biggest contributors after the United States.

It also agreed to support Kabul with annual development aid worth 430 million euros ($529 million), more than any other country.

As part of the NATO-led mission in Afghanistan, Germany has had up to 5,000 soldiers stationed in the country. Some 55 German soldiers have being killed in combat.

Civilian and military deaths have risen in 2014, the bloodiest year since the war against Taliban militants began.

Afghanistan is in talks with neighbouring countries including China on a regional agreement on peace and security, Ghani said.

"That is all at an early stage. We are working on a detailed plan," he added.

Last month Reuters reported exclusively that China had proposed setting up a forum to restart stalled peace talks between Afghanistan and Taliban insurgents.

Documents seen by Reuters show that China put forward a proposal for a "peace and reconciliation forum" that Afghan officials said would gather representatives from Afghanistan, Pakistan, China and the Taliban command.