On Sunday, he is expected to meet Hamid Karzai, the Afghan president.
"I look forward to seeing what the situation on the ground is," Obama told reporters who accompanied him to his departure from Andrews Air Force Base on Thursday.
"I want to, obviously, talk to the commanders and get a sense both in Afghanistan and in Baghdad of, you know, what the most, their biggest concerns are, and I want to thank our troops for the heroic work that they've been doing."
Call for action
The senator has said that Afghanistan is the central front in the so-called "war on terror," and he is expected to call for more US action to rout the Taliban and al-Qaeda.
Obama has said that if he wins the elections in November, he will commit at least two more combat brigades, up to 10,000 men, to Afghanistan while downscaling the size of the force in Iraq.
Al Jazeera's James Bays, reporting from Kabul, said that Obama's visit is attracting a lot of attention even among ordinary Afghans.
"His visit is cloaked in secrecy ... during his time here he will meet the Afghan president, Hamid Karzai, although there will be no press conference just an opportunity for the two leaders to speak and pose for photographs."
Bays said that if Obama is elected president and takes office in January 2009, then the Afghan elections will be one of his top priorities because Karzai will also be facing elections in the next year.
Obama left the US on Thursday and stopped first in Kuwait, where he visited troops, Robert Gibbs, a senior aide to the Illinois senator, said in a statement.
John McCain, Obama's Republican rival, has criticised, the Democratic nominee's lack of experience in the region and on Saturday he mocked his current trip.
"Senator Obama announced his strategy for Afghanistan and Iraq before departing on a fact-finding mission that will include visits to both those countries," he said in a radio address.
|Obama says Afghanistan is the central front in the so-called war on terror [AFP]
"Apparently, he's confident enough that he won't find any facts that might change his opinion or alter his strategy. Remarkable."
Speaking on the jet before it landed in Afghanistan, Obama said: "I am there to listen, but there is no doubt that my core position, which is that we need a timetable for withdrawal, not only to relieve pressure on our military but also to deal with the deteriorating situation in Afghanistan and to put more pressure on the Iraqi government."
Obama said that it was time to respect the wishes of Iraq as a sovereign government and start withdrawing troops. A move he said, is in the strategic interests of the US.
In an interview with German news weekly Der Spiegel
on Saturday, Nuri al-Maliki, Iraq's prime minister, backed the Democratic nominee's plan to pullout US troops.
"We feel that this would be the right timescale for withdrawal allowing for minor adjustments," he said in the interview to be published on Monday. US forces should leave the country "as soon as possible," al-Maliki said.
It follows an agreement between him and George Bush, the US president, to set only a "general time horizon" for military withdrawal as part of a long-term security pact.