ADEN (Reuters) - Yemen’s main southern separatist group announced early on Sunday it would establish self-rule in areas under its control, which the Saudi-backed government warned would have “catastrophic consequences”.

The move threatens to renew conflict between the UAE-backed separatist Southern Transitional Council (STC) and the Saudi-backed government, nominal allies in Yemen’s war, even as the United Nations is trying to secure a nationwide truce to confront the novel coronavirus.

The STC deployed its forces on Sunday in Aden, the southern port which is the interim seat of the government ousted from the capital, Sanaa, by the Iran-aligned Houthi movement. Reuters journalists saw STC fighters in a column of pickup trucks and military vehicles riding down a main street in Aden.

The STC is one of the main groups fighting against the Houthis as part of a coalition led by Saudi Arabia. But the separatists, long backed by Saudi coalition partner the United Arab Emirates, have clashed with government forces in the past.

In a statement, the STC announced emergency rule in Aden and all southern governorates, saying it would take control of Aden’s port and airport and other state institutions such as the central bank.

The Saudi-backed government and southern regions of Shabwa, Hadhramout and Socotra, among the few areas under coalition control, issued separate statements rejecting the declaration.

Yemen’s Foreign Minister Mohammed Al-Hadhrami said the STC announcement constituted “a resumption of its armed insurgency” and a “rejection and complete withdrawal from the Riyadh agreement”, a deal which ended a previous stand-off between the separatists and the government last year.

The STC “will bear alone the dangerous and catastrophic consequences for such an announcement”, he said in a statement.

STC Vice-President Hani Ali Brik accused the government of hampering the agreement. In a Twitter post, he reiterated accusations against Hadi’s government of mismanagement and corruption, charges it denies.

CORONAVIRUS CEASEFIRE

Yemen has been mired in violence since the Houthis ousted Hadi’s government from power in Sanaa in late 2014, prompting the Saudi-led coalition to intervene.

The conflict, seen as a proxy war between Saudi Arabia and Iran, has been in a military stalemate for years.

The Houthis still hold most major cities despite fighting that has killed more than 100,000 people. The war has choked supply lines in the poorest Arabian peninsula nation, leaving millions of people on the brink of famine and dependent on international aid.

The Saudi-led coalition has announced a unilateral ceasefire prompted by a U.N. plea to focus on the coronavirus pandemic. It extended the ceasefire on Friday for a month, but the Houthis have not accepted the truce and violence has continued.

While Yemen has reported only one confirmed case of the novel coronavirus, aid groups fear a catastrophic outbreak should it spread among a malnourished population in a country with a shattered health system and little testing.

The United Nations is trying to convene virtual talks to forge a permanent truce, coordinate coronavirus efforts and agree on humanitarian and economic confidence-building measures to restart peace negotiations stalled since late 2018.

The STC, which has said it wants to be included in any political negotiations, in January pulled out of committees implementing the Riyadh deal.

The UAE, which like the STC opposes the Islamist Islah party that forms the backbone of Hadi’s government, largely scaled down its presence in the war last year, but retains influence through the thousands of southern fighters it backs.