"As of a few hours ago, we did not yet have official confirmation that the Iranian government or an instrument of the Iranian government were holding the three missing Americans," Clinton said during a media availability with Jordanian Foreign Minister Nasser Judeh in Washington.
The three hikers crossed the unmarked border from northern Iraq into Iran, where they were detained by Iranian soldiers, Iranian news agencies reported.
Clinton said U.S. officials asked Swiss diplomats -- who represent American interests in Iran -- "to please pursue our inquiries to determine the status of the three missing Americans." The Swiss represent American interests because the United States and Iran have no direct diplomatic relationship.
"We want this matter brought to a resolution as soon as possible," Clinton said. "And we call on the Iranian government to help us determine the whereabouts of the three missing Americans and return them as quickly as possible."
A fourth hiker in the group who had been ill and stayed behind in Iraq was taken to the U.S. Embassy in Baghdad after his friends were detained, CNN reported.
Notable deaths of 2014 [PHOTOS]