Kabul, Afghanistan (CNN) -- U.S. Defense Secretary Robert Gates arrived in Helmand province in southern Afghanistan Tuesday morning as he continued his two-day trip to the country, meeting with senior officers and Afghan leaders.
Gates' visit is his 13th to Afghanistan since he became defense secretary in 2006.
He last traveled to the embattled country three months ago, in December 2010. He has said he prefers to make quarterly tours to monitor the war effort, according to Pentagon Press Secretary Geoff Morrell.
On Monday, Gates offered his personal apology to Afghanistan President Hamid Karzai for the killings of nine Afghan boys last week in a NATO-led helicopter attack targeting insurgents, saying the incident "breaks our hearts."
Karzai responded at the joint appearance with the visiting Gates by saying he accepted the apology, which appeared to ease for now renewed tension over the issue of civilian casualties caused by the U.S.-led NATO forces fighting Taliban insurgents in Afghanistan.
"While we take the apology with a lot of respect, agree with it and accept it today, I would request Secretary Gates that you take the plea of the Afghan people to Washington that these civilian casualties stop and make the utmost effort so that we don't have them anymore," Karzai said.
The remarks were intended to show that Karzai and his government were moving past the incident despite public anger over the killings and the continuing presence of the U.S.-led military force.
Gates cited that anger in his apology, saying that the deaths of the boys, who were cutting wood, was "a tragedy for their families" and "a setback for our relationship with the Afghan people whose security is our chief concern."
"I would also like to offer President Karzai my personal apology, because I know these tragedies weigh heavily on his heart and create problems for him as the leader and protector of the Afghan people," Gates continued.
From Afghanistan, Gates will fly to Stuttgart, Germany, to visit the U.S. Africa Command and will continue to Brussels, Belgium, to attend a meeting of NATO defense ministers.
CNN's Rick Martin contributed to this report.