An AU spokesperson confirmed that a mission had been denied access, while ECOWAS declined to comment.
Niger’s junta leaders have rejected a diplomatic mission from African countries aimed at restoring constitutional order after a July 26 coup, resisting pressure from the United States and the United Nations to come to the negotiating table.
The African Union (AU) planned to send a joint mission with representatives of the UN and ECOWAS to Niger on Tuesday, but it was denied permission by the junta, which has closed Niger's airspace, French magazine Jeune Afrique reported.
An AU spokesperson confirmed that a mission had been denied access, while ECOWAS declined to comment, Reuters reports.
The junta had already snubbed meetings with a senior U.S. envoy and another ECOWAS delegation that tried to negotiate.
Niger is the world's seventh-biggest producer of uranium, the most widely used fuel for nuclear energy, adding to its strategic importance.
"There's no doubt that diplomacy is the best way to resolve this situation," U.S. Secretary of State Antony Blinken told French radio station RFI on Tuesday.
He said the United States was backing ECOWAS efforts to restore order. He declined to comment on the future of some 1,100 U.S. troops in Niger, where French, German and Italian troops are also stationed.
The United Nations said Secretary-General Antonio Guterres was strongly in support of mediation efforts by ECOWAS.
ECOWAS defence chiefs agreed on Friday on a possible military action plan, which heads of state are expected to weigh up at their summit in the Nigerian capital Abuja.
The West African bloc has imposed sanctions on Niger and Western allies have suspended aid.
U.S. Acting Deputy Secretary of State Victoria Nuland flew to Niamey on Monday but was denied permission to meet with coup leader Abdourahamane Tiani or with Bazoum, who is in detention.
Instead, she spoke for two hours with other army officers.
"These conversations were extremely frank and at times quite difficult, because, again, we're pushing for a negotiated solution... They are quite firm in their view of how they want to proceed, and it does not comport with the Constitution of Niger," Nuland told reporters.
Last week, ECOWAS sent a mission to Niamey led by Abdulsalami Abubakar, a former military ruler of Nigeria, but Tiani also refused to see him.
In contrast, Tiani met on Monday with a joint delegation from Mali and Burkina Faso, both neighbouring countries where the military has seized power from civilians. The juntas there have pledged support for the coup in Niger.
"We will not accept military intervention in Niger. Our survival depends on it," said Abdoulaye Maiga, a spokesman for Mali's junta, appearing on Niger state television.
Heads of state from the Economic Community of West African States (ECOWAS) are preparing for a summit on Thursday to discuss their standoff with the Niger junta, which defied an August 6 deadline to reinstate ousted President Mohamed Bazoum.
The possibility of military intervention will be discussed, but ECOWAS has said it is a last resort.
Residents of Niamey who spoke to Reuters were strongly supportive of the coup.
"I think it will help us fight terrorism more effectively, and pool our forces," said resident Abdoul Aziz Mahamane.
Some pro-coup demonstrators in Niamey have held up Russian flags. Residents and fabric vendors said the flags were in fashion.
"I'm a fan of the Russian flag, which is why I've come today to buy fabrics for the tailor to make me a flag," said Okacha Abdoul-Aziz. "I like Russia because most African countries are with the Russians."
Western allies fear that Niger could go the way of Mali, which threw out French troops and U.N. peacekeepers and invited in mercenaries from Russia's Wagner group after a junta took control in 2021.