Nigeria has accused Mali, Burkina Faso and Niger of letting their people down by quitting the Economic Community of West African States (Ecowas).
It is the first comment from the chair of the economic and political body since the three governments announced their decision to leave on Sunday.
The three states were already suspended from Ecowas after recent coups.
Their military rulers have distanced themselves from ex-colonial power France and strengthened ties to Russia.
Arguing that they wanted to restore security before organising elections, they formed a mutual defence pact in September called the Alliance of Sahel States.
They accuse Ecowas of being influenced by external powers and failing to help them tackle the jihadist violence in their countries.
Withdrawal will hit hard given that the community guarantees visa-free travel and the right of settlement and work in member countries, according to an analysis by AFP news agency.
Mali, Burkina Faso and Niger were all founding members when Ecowas was set up nearly 50 years ago.
Relations between the three countries and the with Ecowas had been tense after coups took place in Niger in July, Burkina Faso in 2022 and Mali in 2020. Ecowas called on all three countries to return to civilian rule.
In its statement on the three countries' departure, Nigeria's foreign ministry accused them of not acting in good faith and criticised their military leaders.
"Unelected leaders engage in a public posturing to deny their people the sovereign right to make fundamental choices over their freedom of movement, freedom to trade and freedom to choose their own leaders," it said.
The ministry added that Nigeria, which is by far the biggest member of Ecowas and is Niger's main economic partner, remained willing to engage with the three countries.
Last week, Burkina Faso announced it had received 25,000 tonnes of free wheat from Russia, which re-opened its embassy there in December. The embassy had been closed since the collapse of the Soviet Union.
Russia, condemned by Western states for waging war in Ukraine, has been seeking to forge new alliances in Africa and elsewhere.
But it is just one of several major actors now stepping up efforts to court political and economic influence in Africa alongside not only China but also India, Turkey, the Gulf states and South Korea as well as Western nations and Japan.