Washington, Jun 25 (IANS): President Barack Obama says to achieve its mission in Afghanistan to dismantle and destroy Al Qaeda and its affiliates, the US must ensure a stable Afghan and an effective Pakistani government.

A day after replacing the top American general in Afghanistan, he also made it clear that the change did not indicate a change in policy and troops could remain in significant numbers in Afghanistan well after his withdrawal timeline begins next summer.

"Our mission, first and foremost, is to dismantle and destroy Al Qaeda and its affiliates so that they can't attack the United States," he said Thursday facing the press for the first time since sacking Gen. Stanley McChrystal.

"The reason we're there in the first place is because 3,000 Americans were killed from an attack launched in that region. We are not going to have that repeated," he said at a joint White House press conference with Russian President Dmitry Medvedev.

"In order to achieve that, we have to make sure that we have a stable Afghan government, and we also have to make sure that we've got a Pakistani government that is working effectively with us to dismantle these networks," he said.

Though his plan calls for the start of a troop withdrawal in a year, "We did not say, starting in July 2011, suddenly there will be no troops from the United States or allied countries in Afghanistan," Obama said.

"We didn't say we'd be switching off the lights and closing the door behind us," Obama said. "We said we'd begin a transition phase that would allow the Afghan government to take more and more responsibility."

Referring to McChrystal's contemptuous remarks about Obama and senior administration officials in a magazine profile story, the president added he is aware of tensions between military commanders and diplomats charged with helping rebuild the country.

But he said replacing McChrystal with Gen. David Petraeus, who successfully led the US military in Iraq, is not a sign of further shakeups to come. It does mean that he expects his civilian and military teams to work together on Afghanistan.

"I am confident that we've got a team in place that can execute. Now I'm paying very close attention to make sure they can execute, and I will be insisting on extraordinary performance moving forward," he said. "Our team has got to be moving forward in synch."