Victoria Nuland, acting US deputy secretary of state, said on Monday that she had met Niger's military leaders but had made no immediate progress in reversing a coup.
"These conversations were extremely frank and at times quite difficult," Ms Nuland told reporters by telephone from the West African nation's capital Niamey.
'We were pushing for negotiated solution. It was not easy to get traction there."
She said that the military leaders were "quite firm in their view on how they want to proceed, and it does not comport with the constitution of Niger".
The visit shows that, almost two weeks after military leaders seized control of Niger and deposed its leader, Washington still believes the situation can be reversed.
Ms Nuland said that the US asked if she and her team could meet Mr Bazoum, but their request "was never granted".
“The window of opportunity is definitely still open,” US State Department spokesman Matthew Miller said earlier on Monday.
Mr Miller said he did not want to estimate how long the window for reversal would remain open but added that the US was pushing for “diplomacy to achieve this objective”.
“We believe the junta should step aside and allow President [Mohamed] Bazoum to resume his duties," he said.
Niger’s military has closed its airspace due to a potential military intervention by neighbouring countries.
The Economic Community of West African States, or Ecowas, had given the junta until Sunday to hand power back to Mr Bazoum, who is under house arrest.
Ecowas has not ruled out the possibility of using military force in Niger, but has said that would be a last resort.
The regional bloc has imposed heavy sanctions on Niger, and neighbouring Nigeria, which supplies the country with about 70 per cent of its power, has cut electricity supplies.
Washington, meanwhile, has “paused” more than $100 million in aid to Niamey.
“For the time being, that assistance will affect development aid to the government, security aid to the government,” Mr Miller said.
“It's a significant amount.”
Last week, Washington suspended counter-terrorism training in Niger. The country is an important ally for the US and other western nations that are working to battle extremist groups in the region.