Victoria Nuland, a veteran envoy and acting deputy secretary of state, said she met for more than two hours with military chiefs who ousted democratically elected Western ally Mohamed Bazoum on July 26.
Nuland's trip, conducted in secrecy until she left, came after the expiration of a deadline set by ECOWAS to reinstate Bazoum by midnight (2300 GMT) on Sunday or risk military intervention.
The 15-nation bloc is reconvening for its own diplomatic push on the crisis with a summit Thursday in Nigerian capital Abuja.
A source close to ECOWAS said an immediate military intervention to restore Bazoum was not envisaged at this stage, adding that the path to dialogue still appeared open.
Speaking to reporters before her departure, Nuland described her talks as "extremely frank and at times quite difficult".
She said she offered the coup leaders "a number of options" to exit the crisis and restore the relationship with the United States, which like other Western nations has suspended aid.
"I would not say that we were in any way taken up on that offer," she said.
She added that the coup leaders did not respond to her requests to meet Niger's self-proclaimed new leader, General Abdourahamane Tiani, or the detained elected president, Mohamed Bazoum, although US officials have been in touch with Bazoum by telephone.
Nuland said she met Brigadier General Moussa Salaou Barmou, who has been named the new military chief of staff and who has worked closely in the past with the United States, which along with former colonial power France has based anti-jihadist operations in the Sahel out of Niger.
Nuland said she warned Niger against following neighbouring Mali in bringing in Russia's Wagner mercenaries.
"The people who have taken this action here understand very well the risks to their sovereignty when Wagner is invited in," said Nuland, who is known for her hawkish stance on Russia.
US Secretary of State Antony Blinken stressed on Monday the need for the international community to find a diplomatic route out of the coup.
"Diplomacy is certainly the preferred way of resolving this situation," he told French Radio RFI.
"It is ECOWAS' current approach. It is our approach."
Niger's neighbours have strongly rejected the possibility of a regional military intervention.
Mali said it and Burkina Faso -- which have both been suspended from ECOWAS over their own military coups -- were sending a joint official delegation to Niamey to show "solidarity (with) the people of Niger".
They have said military intervention would be tantamount to a declaration of war.
Algeria, which shares a long land border with Niger, has also cautioned against a military solution, which President Abdelmadjid Tebboune said would be "a direct threat" to his North African country.
Senators in regional heavyweight Nigeria urged everyone to focus on the "political and diplomatic option".
More envoys coming
Just before the ultimatum expired on Sunday, Niger's military rulers closed the Sahel country's airspace and warned any attempt to enter it would meet with an "energetic and immediate response".
They said there had been a "pre-deployment in preparation for intervention" made by two Central African countries, without naming them, and warned: "Any state involved will be considered co-belligerent."
But Niger's military rulers also urged an ECOWAS delegation to return for talks, Prime Minister Ouhoumoudou Mahamadou told French broadcaster TV5 Monde Monday, after an abortive first attempt.
Mahamadou said in his interview that Bazoum -- held with his wife and son -- was enduring deteriorating conditions.
"The junta have asked the ECOWAS delegation to return" and its members "will be in Niamey probably today (Monday) or tomorrow", Mahamadou said.
An ECOWAS delegation arrived in the capital Niamey last Thursday but did not stay overnight as scheduled. It did not meet either Bazoum or Tiani.
The UN Secretary-General's representative for West Africa and the Sahel, Leonardo Santos Simao, was also in Abuja for talks on the crisis.
France, with which Niger's new rulers have broken military ties, said it would "firmly" back whatever course of action ECOWAS took after the deadline expired.
On Monday, the military leaders appointed former finance minister Ali Mahaman Lamine Zeine as the new prime minister and Habibou Assoumane as the new commander of the presidential guard.
Niger's coup is the latest of several in Africa's Sahel belt since 2020.
Niger has been critical to Western strategies to combat jihadist insurgencies that have plagued the Sahel since 2012, with France and the United States stationing around 1,500 and 1,000 troops in the country respectively.