The African Union on Tuesday expressed "deep regret" over the decision by the authorities in Burkina Faso, Mali, and Niger to withdraw from the West African bloc ECOWAS.

The AU said its commission president, Moussa Faki Mahamat, "calls on regional leaders to intensify the dialogue between the ECOWAS leadership and the three aforementioned countries," which accused the West African bloc of posing a threat to their sovereignty.

The three countries, which have shared tense ties with the Economic Community of West African States (ECOWAS) since the military coups that toppled the previous governments, announced their decision in a joint statement on Sunday.

Burkina Faso, Mali and Niger were founding members of ECOWAS in 1975, but the bloc suspended all three and imposed sanctions on them following the coups.

It also left open a possible military invasion to "re-establish constitutional order" in Niger as a final option if needed.

The United States, which has vocally opposed the coups in the three countries, called on Burkina Faso, Mali and Niger to "continue to work with ECOWAS."

"We continue to support the efforts of ECOWAS and member states to bring peace, security and prosperity to the region," State Department spokesman Matthew Miller told reporters in Washington.

"We would call on all of them to return on the path to democracy, in addition to continuing to engage with ECOWAS," he said, days after Secretary of State Antony Blinken discussed the coups on a trip to Africa.

The coups took place in Niger in July last year, Burkina Faso in 2022 and Mali in 2020.

On Sunday, the trio -- which have formed an Alliance of Sahel States (AES) -- jointly said ECOWAS had come "under the influence of foreign powers, betraying its founding principles" and was a "threat to member states and peoples."

Niger has now sent ECOWAS official notification of its withdrawal from the West African bloc, an official source told AFP on Tuesday, a day after Mali and Burkina Faso did the same.

Nigerien authorities have not revealed the contents of the "note verbale", attached to a letter sent to ECOWAS.

The notification marks the start of a one-year period before withdrawal from the bloc takes effect.

During that time, countries remain bound by their membership obligations, under the bloc's statutes.

On Monday, Mali and Burkina Faso said they had sent ECOWAS "formal notice" of their withdrawal.

The AU said Tuesday that the African Union Commission was willing "to provide all the assistance in its power for the success of the logic of fraternal dialogue, far from all external interference from wherever they come."