(Reuters) - West African nations imposed sanctions and threatened force on Sunday if Niger's coup leaders fail to reinstate ousted President Mohammed Bazoum within a week, while supporters of the junta attacked the French embassy in Niamey.

The 15-nation ECOWAS bloc's response to the Sahel region's seventh coup of recent years came as crowds in Niger's capital Niamey burned French flags and stoned the former colonial power's mission, drawing tear gas from police.

Images showed fires at the embassy walls and people being loaded into ambulances with bloodied legs.

At an emergency summit in Nigeria to discuss last week's coup, leaders of the Economic Community of West African States called for constitutional order to be restored, warning of reprisals if not.

"Such measures may include the use of force," their communique said, adding that defence officials would meet immediately to that effect.

Chad's President Mahamat Idriss Deby, who came to power in 2021 after a coup, met his Nigerian counterpart Bola Tinubu on the sidelines of the summit and volunteered to speak to the military leaders in Niger, two presidential aides told Reuters, asking not to be identified.

Niger's state TV showed Deby arriving and meeting them.

ECOWAS and the eight-member West African Economic and Monetary Union said that with immediate effect borders with Niger would be closed, commercial flights banned, financial transactions halted, national assets frozen and aid ended.

Military officials involved in the coup would be banned from travelling and have their assets frozen, it added.

Niger's prime minister under Bazoum's government, Ouhoumoudou Mahamadou, said ECOWAS sanctions would be disastrous because the country relies heavily on international partners to cover its budgetary needs.

"I know the fragility of Niger, I know the economic and financial context of Niger having been the finance minister and now prime minister," Mahamadou, who was abroad when the coup occurred, told France24 television from Paris.

"This is a country that will not be able to resist these kinds of sanctions. It will be catastrophic."

U.S. Secretary of State Antony Blinken welcomed ECOWAS's action on Sunday.

"We join ECOWAS and regional leaders in calling for the immediate release of President Mohamed Bazoum and his family and the restoration of all state functions to the legitimate, democratically-elected government," said Blinken in a statement.