Yemen's separatist Southern Transitional Council (STC) early on Sunday announced it would establish self-rule in regions under their control, which the Saudi-backed government warned would have "catastrophic consequences".
The move threatens to renew the conflict between nominal allies in Yemen's multifaceted war as the United Nations is trying to secure a permanent truce to combat the coronavirus pandemic in a country weakened by hunger and disease.
Reuters witnesses reported the deployment of STC armed forces in Aden.
In a statement, the STC announced emergency rules in Aden and all southern governorates.
Last year, the STC turned on the government of Abd-Rabbu Mansour Hadi and seized Aden, with violence spilling over to other southern areas.
"The announcement by the so-called transitional council of its intention to establish a southern administration is a resumption of its armed insurgency ... and an announcement of its rejection and complete withdrawal from the Riyadh agreement," Yemen's Foreign Minister Mohammed Al-Hadhrami said in a ministry statement on Twitter.
The STC "will bear alone the dangerous and catastrophic consequences for such an announcement", the statement said.