* Hagel on first trip abroad as U.S. defense secretary
* Last visit to Afghanistan was with then-Senator Obama
* Suicide bomb killed nine outside Afghan defense ministry
By Phil Stewart
KABUL, March 9 (Reuters) - Chuck Hagel's first full day in Afghanistan as U.S. defense secretary began with the sound of a suicide bombing about a kilometre away from one of his morning briefings.
"I wasn't sure what it was," Hagel said, asked about his initial reaction to the blast that killed nine civilians outside the Afghan defense ministry.
"But we're in a war zone. I've been in war ... So (we) shouldn't be surprised when a bomb goes off or there's an explosion."
Hagel's morning briefing pressed on - even as an announcement about the incident came over the loudspeakers at the NATO facility hosting him at the time, aides said.
He would later board a flight to Bagram airfield near Kabul to meet commanders helping run America's longest war, and then fly to an airfield in the eastern city of Jalalabad.
There, he pinned Purple Heart medals on two soldiers who, like him, were wounded in battle.
"It is true. I was in the United States Army in 1968 - Vietnam," he told troops in Jalalabad on the warm day, an American flag hanging from a banner above him.
Hagel, the first Vietnam veteran to become defense secretary, was awarded two Purple Hearts during that conflict and still carries bits of shrapnel in his chest.
For the 66-year-old former Republican senator, the trip is a re-introduction to the Afghan war - one that will be scrutinized by Republican critics who opposed his nomination and questioned his judgment.
The last time Hagel saw the Afghan conflict up close was during a trip with then-Senator Barack Obama in 2008.
Since then, more than 30,000 American "surge" troops have come and gone, and the Democratic president announced last month that about half of the 66,000 U.S. forces remaining will be home by early next year. NATO will wrap up the combat mission by the end of 2014, leaving just a relatively small training and counter-terrorism force.
But even as the war winds down, and NATO commanders focus on shifting the business of war to Afghan forces, the still-resilient Taliban insurgency is making its presence known through high-profile attacks like Saturday's bombing.
Hagel acknowledged as much in a message to NATO personnel upon his arrival on Friday evening.
"Even as we move into more of a support role, this remains a dangerous and difficult mission," Hagel said. "We are still at war and many of you will continue to experience the ugly reality of combat and the heat of battle."
Sergeant Jeremyah Williams, one of the two soldiers who received the Purple Heart from Hagel, was injured on his fifth deployment in the past decade. Williams was on guard duty on Dec. 2 at the time of a suicide bombing just 30 metres (100 feet) from his position at a gate to his base.
Williams said he suffered traumatic brain injury, one of the signature wounds of the Afghan and Iraq wars.
"I was just a little confused about what happened at first," Williams said, adding the blast did not knock him out but made him dizzy, with ringing in his ears.
Obama has trumpeted Hagel's qualifications and war record, noting he fought at the enlisted rank, not as an officer. Hagel, Obama argued, looked at war with the perspective of "the guy at the bottom" sent to fight, and perhaps die, abroad.
Williams expressed pride at receiving the Purple Heart from Hagel. But he did not seem to care much whether Hagel had been an officer, or not.
"It really doesn't matter if you're enlisted or an officer - we're all really here to do the same job," he said. (Additional reporting by Mirwais Harooni; Editing by Vicki Allen)