The BBC reported on its web site that Eritrean President Isaias Afewerki had refused to allow the US delegation, led by Assistant Secretary of State for African Affairs Jendayi Frazer, into his country.
There was immediate word on whether Ms Frazer was scrapping her planned effort to help resolve Eritrea's boundary dispute with Ethiopia left over from a peace deal struck to end their 1998-2000 war.
"The schedule is still in flux," said a senior US State Department official. He could not confirm that Mr Afewerki had barred Ms Frazer from Eritrea but said she had her visa to enter the country.
"Assistant Secretary Frazer intends to visit the border region but the final details of the intinerary are still being worked out," the US official said.
Eritrea had last week expressed its misgivings about the mission by Ms Frazer, who was to have been accompanied by Marine General Carlton Fulford on visits to Asmara and Addis Ababa.
The Eritrean information ministry doubted the "legality" and "political relevance" of the mission and said Asmara would welcome only those initiatives that forced Ethiopia to accept a 2002 border ruling.
Ethiopia has demanded revisions to the border demarcation decided by an international panel that Addis Ababa had previously accepted as part of efforts to end the conflict that cost some 80,000 lives.
The planned US mediation came as the UN Security Council mulled the future of its peacekeeping troops on the border who have been severely hampered by restrictions slapped on them by Eritrea.
Ms Frazer flew to Africa with First Lady Laura Bush to attend the inauguration Monday of Liberian President Ellen Johnson Sirleaf.