US President Barack Obama has declared that the US combat role in Afghanistan is winding down just as it has already ended in Iraq.

"We can see the light of a new day on the horizon," he said at the end of a surprise trip to the country on the anniversary of Osama bin Laden's death.

Shortly after Obama's visit, a series of explosions hit the capital Kabul early on Wednesday, with reports of several people being killed.

"Our goal is to destroy al-Qaeda, and we are on a path to do exactly that," Obama said in his speech on Tuesday, broadcast from the Bagram air base to an American audience halfway around the world.

He spoke after signing an agreement with Afghan President Hamid Karzai setting post-war promises and expectations.

With two armoured troop carriers as a backdrop, Obama made his remarks in the midst of his endeavour to win re-election as US president and commander-in-chief.

He said that he would keep up the steady withdrawal of troops from Afghanistan and that there was a "clear path to fulfill our mission" after more than a decade of military involvement there.

"My fellow Americans, we have travelled through more than a decade under the dark cloud of war. Yet here, in the pre-dawn darkness of Afghanistan, we can see the light of a new day on the horizon," he said.

"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."

'Strong words'

Speaking to Al Jazeera's Patty Culhane, Wahid Monawar, former Afghan ambassador to the UN atomic agency, said Obama used "strong words" in his speech.

"The president was really clear. This is a huge day for the Afghan people," Monawar said.

"To reach the agreement, it went through a lot of impediments. There was the issue of the transfer of detainees to Afghan responsibility, the night raids.

"All of that was overcome by the two governments, so I can definitely congratulate the Afghan government for reaching the agreement."
   
Obama's unannounced visit in the midst of his 2012 re-election campaign appeared aimed at showing US voters he is pursuing a strategy to wind down the war.

He was also seeking to reassure Afghans that Washington was not abandoning them in the face of a continuing Taliban insurgency. Most US and NATO troops are due to leave in 2014.

Accord signed

Earlier in the day, Obama and Karzai signed a strategic partnership accord that charts the future of US-Afghan relations beyond the end of the NATO combat mission in the country in 2014.

The partnership spells out the US relationship with Afghanistan beyond 2014, covering security, economics and governance.

The deal is limited in scope and essentially gives both sides political cover: Afghanistan is guaranteed its sovereignty and promised it won't be abandoned, while the US gets to end its combat mission in the long and unpopular war but keep a foothold in the country.

The deal does not commit the US to any specific troop presence or spending, but it does allow the US to potentially keep troops in Afghanistan after the war ends for two specific purposes: continued training of Afghan forces and targeted operations against al-Qaeda.

The group is present in neighbouring Pakistan, but has only a nominal presence inside Afghanistan.