West African military chiefs are holding a three-day meeting in Nigeria's capital Abuja to discuss how to respond to a July 26 coup in neighbouring Niger.
The coup has raised fears of a regional conflict and prompted France, the former colonial power, to fly out its citizens.
Ecowas, an alliance of West African countries that is currently led by Nigeria, on Sunday imposed trade and financial sanctions and gave the coup leaders a week to reinstate Niger's democratically elected president, Mohamed Bazoum, or possibly face force.
The self-proclaimed new leader of Niger, Abdourahamane Tiani, on Wednesday rejected the sanctions imposed by the West African regional bloc as illegal, unjust and inhumane.
In a televised address, Mr Tiani said Niger would not bow to regional and international pressure to reinstate Mr Bazoum.
The West African regional bloc on Wednesday said a military intervention in junta-ruled Niger would be "the last resort".
"The military option is the very last option on the table … but we have to prepare for the eventuality," said Abdel-Fatau Musah, the Ecowas commissioner for political affairs, peace and security.
An Ecowas delegation is also in Niger to speak directly to the junta leaders.
The delegation is led by former Nigerian president Abdulsalami Abubakar and includes a highly respected Nigerian religious leader, the Sultan of Sokoto, Muhammadu Sa'ad Abubakar.
Nigeria, which has the fourth largest army in Africa, has led the Ecowas response to the coup.
Analysts have told The National they believe that if mediation attempts fail, a Nigerian-led military intervention is likely.
There is little expectation that any western country would be directly involved.
British Prime Minister Rishi Sunak's office said he had spoken with Germany's Chancellor Olaf Scholz to discuss the situation in Niger.
“The UK and Germany have both condemned the recent attempts to undermine democracy, peace and stability in the country,” the readout of the call said.
“The Prime Minister welcomed our shared efforts to uphold stability and protect our nationals in Niger.”
Italy's Foreign Minister Antonio Tajani told media on Wednesday that: “We have to work so that democracy prevails in Niger … we need to rule out any western military initiative because it would be perceived as a new colonisation.”
Ecowas's firm response to the coup has been welcomed by western leaders.
“We stand ready to support Ecowas’s upcoming decisions, including the adoption of sanctions,” the EU's top diplomat Josep Borrell said on Saturday.
Russia on Wednesday called for “urgent national dialogue” in Niger and cautioned that the situation could worsen.
“It's very important to prevent a further deterioration of the situation in the country,” Russian Foreign Ministry spokeswoman Maria Zakharova told reporters.
Ms Zakharova said dialogue was needed for the “restoration of civil peace and to ensure law and order”.
But the threat of a military intervention was met by a strong rebuttal from Mali and Burkina Faso, both led by military juntas, who said that it would be equal to a declaration of war against them.
Niamey, Bamako and Ouagadougou seem to have been co-ordinating their response to Ecowas, with Gen Salifou Mody, one of the officers who seized power in Niger last week, visiting Mali on Wednesday.
Gen Mody, a former army chief of staff who was fired in April, arrived in Bamako at the head of a delegation, a senior Nigerien official and a Malian security official told AFP.
The coup leaders announced overnight they had reopened Niger's land and air borders with Algeria, Burkina Faso, Mali, Libya and Chad.
On Wednesday more than 730 people had landed in Paris aboard three planes, mostly French citizens but also Portuguese, Belgians, Nigerians, Ethiopians and Lebanese, with a fourth flight scheduled.
Italy's Foreign Ministry said it had flown out 68 civilians, including Italians and other nationals living in Niger, who arrived in Rome early Wednesday.
Eighteen Italian soldiers were also on the flight.
“It feels good,” said a relieved Raissa Kelembho, who returned from Niger with her two boys, but whose husband stayed behind to work.
“At one point, there was a feeling of insecurity, we knew that everything could change."
Western soldiers in Niger as part of anti-insurgency combat missions remain in the country. They include about 1,500 French troops, 1,100 US troops and a small German contingent.
German Defence Minister Boris Pistorius said on Wednesday that there were no concerns about German troops in Niger, and that supply was secure.
“I spoke yesterday with the commander of our airbase in Niamey,” Mr Pistorius said. “He clearly told me that they are not concerned about their safety at all currently.”
But Germany has urged its citizens to leave the country.
Niger evacuation flights land in Europe – in pictures
The first group of British citizens left Niger on a French flight bound for Paris on Wednesday evening, a Foreign Office representative said.
The representative did not say how many Britons were on the flight.
“The UK’s ambassador and a core team remain in Niger to support the very small number of British nationals who are still there,” their statement said.
“We are grateful to the French for their help in this evacuation.”
UK Foreign Secretary James Cleverly said on social media: "The first group of British nationals have now safely left Niger.
"To resolve the situation in Niger, the UK is clear in our support of an African and Ecowas-led resolution.
"This week I’ve spoken with the presidents of Ghana and Nigeria to support this."
The US said on Wednesday it was set to evacuate some staff and families from its embassy, although the mission will remain open and senior leadership will continue working from there.
The official said a final decision was still pending but it appeared likely.
“This is a prudent move in case the security situation deteriorates and the core embassy staff will remain,” the official said.
They said the US personnel would be moved out of Niger by aircraft chartered by the State Department and military aircraft would not be used.