The EU has suspended all security cooperation with Niger after the country's army took power in a coup.
It comes shortly after the US declared its "unflagging support" for ousted president Mohamed Bazoum - seen as a key Western ally in the fight against Islamist militants.
He said insecurity, economic woes and corruption led him to seize power.
But there are now concerns in the West about which countries the new leader will align with.
Niger's neighbours, Burkina Faso and Mali, have both pivoted towards Russia since their own coups.
EU foreign policy chief Josep Borrell joined the US and France in refusing to recognise the coup leaders and said security cooperation and budgetary aid was being suspended indefinitely.
Also on Saturday the African Union called on the Niger army to return to base within 15 days.
On Friday evening US Secretary of State Antony Blinken warned those detaining him that "hundreds of millions of dollars of assistance" was at risk.
However, the leader of Russia's Wagner mercenary group has reportedly described the coup as a triumph.
"What happened in Niger is nothing other than the struggle of the people of Niger with their colonisers," Yevgeny Prigozhin was quoted as saying on a Wagner-affiliated Telegram channel.
"With colonisers who are trying to foist their rules of life on them and their conditions and keep them in the state that Africa was in hundreds of years ago."
He added: "Today this is effectively gaining their independence."
The BBC has not been able to verify the authenticity of his reported comments.
Wagner is believed to have thousands of fighters in countries including the Central African Republic (CAR) and Mali, where it has lucrative business interests but also bolsters Russia's diplomatic and economic relations.
Wagner fighters have been accused of widespread human rights abuses in several African countries.
Gen Tchiani, 62, has been in charge of the presidential guard since 2011 and was promoted to the rank of general in 2018 by former President Issoufou.
He had also been linked to a 2015 coup attempt against the ex-president, but appeared in court to deny it.
On Friday Gen Tchiani said his junta took over because of problems in Niger including insecurity, economic woes and corruption.
He also addressed Niger's global allies, saying the junta would respect all of the country's international commitments, as well as human rights.
But the junta has had strong words for those who oppose them, accusing members of the ousted government who have taken refuge in foreign embassies of plotting against them.
They said any such attempt would lead to bloodshed, which has so far been avoided.
Life in the capital Niamey has largely returned to normal with markets and shops open, but civil servants have been told to go home.
Meanwhile Nigeriens have mixed feelings about the coup, with some saying insecurity in the country wasn't severe enough to justify a coup. But others have supported the junta.
Niger's coup is the latest in a wave of military takeovers that have hit the West African region in recent years, toppling governments in countries including Mali, Guinea and Burkina Faso.
It also comes as a big blow to the leadership of regional body Ecowas. Just two weeks ago, the bloc's chairman, President Bola Tinubu, warned that terrorism and the emerging pattern of coups in West Africa had reached alarming levels and demanded urgent, concerted actions.
This is the fifth coup in Niger since it gained independence from France in 1960, on top of other unsuccessful takeover attempts.