KABUL - ?The new Pentagon chief Chuck Hagel arrived in Afghan capital of Kabul on Friday on an unannounced visit to the war-torn country, his first trip abroad since taking office as US defense secretary a couple of days ago.

"US Secretary of Defense Chuck Hagel arrived in Afghanistan this evening, his first visit as secretary of defense," the NATO- led International Security Assistance Force (ISAF) said in a statement.

"I believe that we are at a very important moment in this campaign, with the Afghan National Security Forces (ANSF) on the verge of stepping into the lead for all combat operations across the country," Hagel told ISAF service members in his message.

"As the 2013 fighting season gets underway, the ANSF will be doing more and more of the fighting, and relying on you for support, training, and advice," he said.

"What you are doing here in Afghanistan will help ensure that al-Qaida does not re-establish the safe haven they had before September 11, 2001 .. You have my full support, and I will do everything I possibly can to help you succeed and come home safely to your families. I have no higher priority as Secretary of Defense than your safety, success, and welfare," he said in his message to ISAF.

Hagel also emphasized for a smooth transition of security from US and NATO forces to ANSF, saying "Even as we move into more of a support role, this remains a dangerous and difficult mission .. But the goal we have established ?to have Afghans assume full responsibility for security by the end of 2014 ?is clear and achievable,"

Under the US President Barack Obama's withdrawal plan 33,000 US troops already pulled out of Afghanistan in September last year and another 34,000 will return home within one year.

Hours before his arrival, the coalition forces registered the second so-called "green-on-blue" insider attack when Afghan army or police or gunman in their uniform kill their foreign partners.

"Individuals wearing Afghan National Army uniforms turned their weapons against International Security Assistance Force members in eastern Afghanistan today, killing one ISAF-contracted civilian," the ISAF confirmed in a statement.

The incident is currently under investigation, it said.

In January, one British soldier was killed by an Afghan soldier while more than 60 US and NATO troops lost their lives in such attacks in 2012.

Earlier in the day, the US forces also confirmed losing an American soldier, who lost his lives in a non-combat incident in the southern Afghanistan on Thursday.