Barack Obama's government has declared Afghanistan the United States' newest "major non-Nato ally".
The move is designed to aid close defence co-operation after US combat troops withdraw from the country in 2014 and as a political statement of support for Afghanistan's long-term stability.
US secretary of state Hillary Clinton, who arrived in Kabul on an unannounced visit to meet Afghan president Hamid Karzai, disclosed the alliance to diplomats at the US embassy.
The designation allows for streamlined defence co-operation, including faster purchasing ability of American equipment and easier export control regulations.
Afghanistan's military, which is heavily dependent on American and foreign assistance, already enjoys many of these benefits and the non-Nato ally status guarantees it will continue to do so.
"I am going to be announcing formally with President Karzai in just a little bit that President Obama has officially designated Afghanistan as what's called a major non-Nato ally of the United States," Mrs Clinton said.
Afghanistan becomes the 15th such country the US has declared a major non-Nato ally. Others include Australia, Egypt, Israel and Japan. Afghanistan's neighbour Pakistan was the last nation to gain the status in 2004.
The declaration was part of a Strategic Partnership Agreement signed by Mr Obama and Mr Karzai in Kabul at the beginning of May.
On July 4 US ambassador to Afghanistan Ryan Crocker and the country's foreign minister announced that the two countries had completed their internal processes to ratify the agreement, which has now gone into force.
Mrs Clinton and Mr Karzai were expected to discuss US-Afghan civilian and defence ties and stalled Afghan reconciliation efforts. From Kabul, Mrs Clinton is heading to Japan for an international conference on Afghan civilian assistance.