The threat of an invasion, though not specific, raises tensions in and around Niger, a uranium producer that until the coup was an important ally of the West in the fight against Islamist insurgents devastating the Sahel region.
The junta, which seized power on July 26, had defied an August 6 deadline to stand down set by the Economic Community of West African States (ECOWAS), instead closing Niger's airspace and vowing to defend the country against any foreign attack.
After a summit of its heads of state in the Nigerian capital Abuja, the bloc pledged to enforce sanctions, travel bans and asset freezes on those preventing the return to power of democratically elected President Mohamed Bazoum.
"No option is taken off the table, including the use of force as a last resort," said Nigerian President Bola Tinubu, the ECOWAS chair.
"I hope that through our collective effort we can bring about a peaceful resolution as a roadmap to restoring stability and democracy in Niger," he said. "All is not lost yet."
An official statement was read out which included a resolution asking the bloc's defence chiefs to "activate the ECOWAS Standby Force with all its elements immediately".
Reiterating support for the efforts by ECOWAS and a call for the release of President Bazoum, US Secretary of State Antony Blinken said the United States would hold the junta accountable for the safety of Bazoum, his family, and detained members of the government.
"The United States appreciates the determination of ECOWAS to explore all options for the peaceful resolution of the crisis," Blinken said.
Security analysts said an ECOWAS force could take weeks or longer to assemble, potentially leaving room for negotiations.
The bloc has planned to create a standby force of thousands of troops for years but was held back by funding delays and insufficient troop commitments, said Ikemesit Effiong, a researcher at SBM Intelligence in Nigeria.
After a string of coups since 2020 and growing militant activity, regional leaders said in December that they were determined to create such a force.
ECOWAS Commission President Omar Alieu Touray told the UN Security Council last month they were considering two options; a brigade of 5000 troops at an annual cost of $US2.3 billion ($A3.5 billion) or the deployment of troops on demand at an annual cost of $US360 million ($A552 million).
Thursday's statement did not spell out how the force would be funded, which countries would participate or how many troops and what hardware they could contribute.
ECOWAS sought to project an image of resolution and unity, but the bloc is split, with suspended member states Mali and Burkina Faso, also ruled by military governments, vowing to defend the Niger junta.
The United Nations and Western powers have backed ECOWAS efforts to persuade the coup leaders to relinquish power and free Bazoum, who is being detained in his residence, but so far they have given no sign they were willing to back down.
Western countries fear Niger could follow Mali's footsteps and seek help from Russia's Wagner Group, which the US has designated a transnational criminal organisation.
Wagner's chief, Yevgeny Prigozhin, has welcomed the Niger coup and said his forces were available to restore order.
