Billions of dollars in American assistance to Afghanistan could ultimately go to waste without better planning and security in the war-torn nation, a US investigator said Thursday.

John Sopko, the congressionally mandated Special Inspector General for Afghanistan Reconstruction, aired his concerns as Afghan President Hamid Karzai held talks in Washington to prepare for next year's withdrawal of US combat troops.

"We are at a risk now of wasting billions of dollars if the agencies charged with implementing new programs and constructing new facilities do not first answer some basic questions," Sopko said at the Stimson Center think tank.

Despite US commanders' talk of progress in Afghanistan, Sopko said that it was becoming more difficult for inspectors to head into the field to ascertain that US money was being well spent.

"As the military draws down, we too find that there are fewer places that we can go to safely in Afghanistan to monitor projects," he said.

Sopko accused both the Pentagon and the US Agency for International Development of lacking long-range plans on where and why they were building projects and of often failing to ensure quality standards.

He pointed to a $70 million US-funded garrison for Afghan troops in northern Kunduz province that was rendered unusable because it was built on unstable soil with roofs that collapsed due to improper welding.

The United States has appropriated more than $90 billion for Afghanistan's reconstruction since 2001, when a US-led coalition ousted the hardline Taliban regime following the September 11 attacks carried out by Al-Qaeda.

Sopko said the assistance amounted to $28 million a day and was more than the United States has spent on any nation since World War II.

He also pointed to concerns over graft. In its latest annual survey, Berlin-based watchdog Transparency International ranked Afghanistan worst -- in a tie with North Korea and Somalia -- on perceptions of corruption.

President Barack Obama's administration has voiced concern to Karzai over corruption, leading to sometimes rocky relations.

Obama meets Karzai on Friday as the US administration debates how many troops to leave in Afghanistan. Opinion polls have shown that the US public wants to end America's longest war and pull out the 66,000 combat troops.

Some US officials want to keep just a few thousand US troops to train Afghans and take part in operations against Al-Qaeda. The White House has left open the option of withdrawing forces completely.