(CNN) -- Incidents similar to this week's fatal shooting of six U.S. troops should not overshadow the progress American forces have made in turning security over to the Afghans, NATO's supreme commander says.
The six were shot Monday by a gunman wearing an Afghan Border Police uniform.
Last summer, two U.S. civilians and an Afghan soldier were reported shot to death by another Afghan soldier. A "rogue" Afghan policeman was blamed for the November 2009 shootings deaths of five British troops in Helmand province.
"Afghanistan is going to be a roller coaster," Adm. James Stavridis, NATO's supreme commander, said Monday. "We're going to see ups and downs. I see gradual, steady progress in Afghanistan, and I remain cautiously optimistic that we're going to succeed in Afghanistan. And I think one of the keys is transition. It is turning security over to the Afghans themselves."
Monday's shooting occurred during a training mission in eastern Afghanistan, the NATO-led International Security Assistance Force said in a news release. The gunman was also killed in the incident, ISAF said.
A joint ISAF and Afghan team is investigating. The military said Tuesday it was awaiting information from its initial assessment team.
"Today we have 265,000 active Afghan police and Afghan army," Stavridis said. "We're going to build that number to about 300,000."
"Are we going to have incidents like the tragedy we had today? Yes, but the great sweep of progress in terms of the improvement of the Afghan security forces continues, and I see it most vividly at the moment in Helmand and Kandahar, where 60 percent of the operations are conducted by Afghans," he said. "That's a major improvement over 18 months ago when I took over. So I think we'll continue to see progress, albeit with the kind of incidents and tragedies that we face today."
Stavridis said he has faith in Afghan President Hamid Karzai's regime to help further that progress.
"I take great stock in President Karzai's presentation at the summit in Lisbon [Portugal], where he pledged to begin the transition to 2011 and continue it to its conclusion, we hope, in 2014" to an Afghan-led operation, he said. "So I am confident in the intent, and I see real progress across the spectrum of training the Afghan security forces."
U.S. President Barack Obama has pledged to start withdrawing some U.S. troops from Afghanistan next July. Officials have said the goal is to end combat operations in Afghanistan in 2014.
CNN's Paul Courson, Samson Desta and Chris Lawrence contributed to this report.