The designation, which does not entail any security commitments, was first announced in May but is now being fully implemented.
Other countries in the category include Pakistan, Japan, South Korea and Israel.
It is one more way that the Obama administration is seeking to reassure Karzai that the United States will not abandon his country after American and NATO troops leave. Another means of reassurance is money, the topic of the Tokyo conference that convenes Sunday.
A U.S. diplomatic official said Saturday that donor nations, many of which are deeply wary of corruption in Afghanistan, had pledged $16 billion in development aid to the country over the next four years, an amount that was more or less expected.
In return, Karzai has pledged new anti-corruption measures.
The arrangements are part of the delicate dance that Karzai has managed in recent years, on the one hand harshly criticizing the conduct of U.S. troops — particularly when civilians have been killed or airstrikes used on Afghan homes — while also pressing for as much American money, supplies and training for his country as he can get for as long as possible.
U.S. officials have yet to negotiate the exact number of troops that will remain in Afghanistan after combat operations are expected to conclude in 2014.
Clinton landed in Kabul early Saturday morning. She addressed U.S. and Afghan employees at the U.S. Embassy, where she was received with cheers and whoops, and then had breakfast with Karzai under the shade of trees in the garden outside his palace.
It was Clinton’s first visit to Kabul since President Obama and Karzai signed an agreement May 1 that commits the United States to supporting Afghanistan for the next decade in security, economic development, rule of law and other areas.
Clinton has enjoyed a relatively close relationship with Karzai, who has often clashed with U.S. commanders and ambassadors over the years.
During a news conference in the garden after their breakfast, Clinton told reporters, “We are not even imagining abandoning Afghanistan.”
Then she got back in the plane and headed to Tokyo.
Joshua Partlow contributed to this report.