Washington has been deliberating for months on revisions to its war strategy for Afghanistan as world leaders weigh their commitment against the legitimacy of the Afghan government.
"We stand at a critical moment on the eve of the inauguration of President Karzai's second term," said Clinton.
Karzai in May sparked controversy when he chose Mohammad Qasim Fahim, a former rebel leader accused of war crimes, as his running mate for the August election.
In another move, Karzai has embraced Uzbek militant leader Gen. Abdul Rashid Dostum, who faces a wide range of charges of abuse and aggressive tactics. The credibility of the Afghan president was tarnished further by allegations of rampant corruption in the August presidential contest.
Afghan officials said they would enact tough new anti-corruption measures amid international pressure to address legitimacy issues.
U.S. officials said that while Washington welcomed the effort to take on issues of corruption, it was the actions of the Afghan government to tackle the matter that counted.
"We'll look forward to getting more details about how it intends to operate, what kind of cases it intends to prosecute," said U.S. State Department spokesman Ian Kelly. "But again, it's the actions that are important, and so we'll be monitoring it closely."
Karzai is scheduled for a swearing-in ceremony in Kabul on Thursday amid tight security.