By Boureima Balima and Abdel-Kader Mazou
NIAMEY (Reuters) -The junta that seized power in Niger last week detained senior politicians on Monday, their party said, defying international calls to restore democratic rule, while fellow military rulers in West Africa expressed their support.
The overthrow of President Mohamed Bazoum has sent shockwaves across West Africa, pitting Niger's former Western allies against the likes of Russia and other junta leaders in the region.
The African Union, the U.N., the European Union and other powers have condemned the overthrow of President Mohamed Bazoum - the seventh military takeover in less than three years in West and Central Africa that has undermined democratic progress in one of the world's poorest regions.
Regional bloc ECOWAS has imposed sanctions, including a halt in all financial transactions and a national assets freeze, and said it could authorise force to reinstate Bazoum, who is still locked in his palace.
But the juntas of neighbouring Burkina Faso, Mali and Guinea all voiced their support for the coup's leaders on Monday.
"Mali and Burkina Faso warn that any military intervention in Niger will be considered as a declaration of war against Burkina Faso and Mali," said a joint statement read on both countries' national broadcasters.
Niger's junta on Monday arrested the ousted government's mines minister, the head of the ruling party and the oil minister, among others.
Meanwhile, a United States official on Monday said the coup had not been fully successful and that there was still an opportunity to reinstate Bazoum. France and Germany echoed those comments.
Last Wednesday's coup has raised fears for the security of the Sahel region. Niger is the world's seventh-biggest producer of uranium, the radioactive metal widely used for nuclear energy and treating cancer.
The United States, former colonial power France and other Western states have troops in Niger and had been working with the government to overcome an Islamist insurgency by groups linked to Islamic State and al Qaeda.
But attacks on civilians and soldiers persist, fomenting discontent and straining relations with Western powers.
There have been four takeovers in neighbouring Mali and Burkina Faso during the last two years, all of which have come amid frustrations about growing insecurity. Both countries have turned increasingly towards Russia as an ally.
The coup leaders, who have named General Abdourahamane Tiani, the former presidential guard chief, as head of state, said they overthrew Bazoum over poor governance and discontent with the way he handled the Islamist threat.
On Sunday, supporters of the junta burned French flags and attacked the French embassy in Niger's capital, Niamey, prompting police to fire volleys of tear gas in response.
Yevgeny Prigozhin, the head of Russia's Wagner mercenary group, last week welcomed the coup in Niger, and said his forces were available to restore order.
The Kremlin said on Monday that the situation in Niger was "cause for serious concern" and called for a swift return to constitutional order.
ECOWAS appears to have taken a tougher stance towards Niger than its junta-led neighbours, which have been sanctioned but never threatened with force.
Both the EU and France have backed the bloc's response suspended their own financial support, while the U.S. has threatened to do so.
"The EU and Niger share deep ties developed over decades. The unacceptable attack on the democratically elected government puts these ties in jeopardy," European Commission President Ursula von der Leyen posted on X, the platform formerly known as Twitter.
After days of turmoil, the International Monetary Fund said it was closely monitoring developments in Niger. But the IMF has not yet taken any specific actions in response to the coup.
It has yet to disburse a $131.5 million loan to Niger that was approved on July 5, it added.
The regional central bank has meanwhile cancelled Niger's planned 30 billion CFA ($51 million) bond issuance, scheduled for Monday in the West African regional debt market, following sanctions, sources said.
(Reporting by Boureima Balima and Abdel-Kader Mazou; Additional reporting by Anait Miridzhanian, Bate Felix, Fadimata Kontao, Thiam Ndiaga; Writing by Nellie Peyton and Sofia Christensen; Editing by Alexander Winning, Bernadette Baum, Andrew Heavens, Nick Macfie and Gerry Doyle)