US President Barack Obama is to tell Americans from Afghanistan that a decade under the "dark cloud" of war was ending and that the "light of a new day" was on the horizon.

"This time of war began in Afghanistan, and this is where it will end," Obama was to say after making a secret trip to sign a pact on post-2014 ties with Kabul on the anniversary of the killing of Osama bin Laden.

"My fellow Americans, we have traveled through more than a decade under the dark cloud of war," Obama said, according to the advance excerpts of remarks he was to make in a major speech from Bagram air base at 2330 GMT Tuesday.

"Yet here, in the pre-dawn darkness of Afghanistan, we can see the light of a new day on the horizon.

"The Iraq War is over. The number of our troops in harm's way has been cut in half, and more will be coming home soon. We have a clear path to fulfil our mission in Afghanistan, while delivering justice to Al-Qaeda."

Obama also carried a finely tuned political message, four days before he holds the official kick-off rallies for his bid for a second White House term in the key battleground states of Ohio and Virginia.

"As we emerge from a decade of conflict abroad and economic crisis at home, it is time to renew America," Obama said.

"A united America of grit and resilience, where sunlight glistens off soaring new towers in downtown Manhattan, and we build our future as one people, as one nation," Obama said.

The president noted that half of Afghans were now living in places where their own security forces were in control and that all 33,000 surge troops he sent to Afghanistan would be home within months.

"After that, reductions will continue at a steady pace, with more of our troops coming home. And as our coalition agreed, by the end of 2014 the Afghans will be fully responsible for the security of their country," Obama said.

He paid tribute to the sacrifices of US men and women in uniform.

"This future is only within reach because of our men and women in uniform," he said.

"I just met with some of them, and told them that as commander-in-chief, I could not be prouder. In their faces, we see what is best in ourselves and our country," Obama was to say, according to remarks released by the White House.