US presidential hopeful Barack Obama called the situation in Afghanistan "precarious and urgent" and said Washington should start planning to transfer more troops there from Iraq.

The Illinois Democrat spoke from Afghanistan after meeting privately with Afghan President Hamid Karzai on the second day of an overseas trip meant to bolster his foreign policy credentials.

"We have to understand that the situation is precarious and urgent here in Afghanistan and I believe this has to be the central focus, the central front, in our battle against terrorism," Obama said.

He said the United States should start planning immediately for a shift of American soldiers from Iraq to Afghanistan.

There are four times as many US troops in Iraq as the 36,000 in Afghanistan, yet more American soldiers were killed in Afghanistan than in Iraq in both May and June.

"I think the situation is getting urgent enough that we have to start doing something now," Obama said.

After arriving in Afghanistan, Obama was briefed by the US commander of NATO-led forces in the east of the country.

He will also visit Iraq, Jordan, Israel, Germany, France and Britain on a foreign tour he hopes will help answer Republican criticism that he lacks the experience to be commander in chief.

His Republican rival in November's US election, John McCain, is a Vietnam War veteran who says he will be stronger on security than the first-term Illinois senator.

More than six years after a US-led coalition ousted the Islamist Taliban government that had sheltered al Qaeda, there has been a sharp rise in violence in Afghanistan this year.

"One of the biggest mistakes we've made strategically after 9/11 was to fail to finish the job here, focus our attention here. We got distracted by Iraq," Obama said.

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