Yemen's president on Saturday called on southern separatists to "stop the bloodshed" and abide by a power-sharing agreement, in his first public comments since the secessionists declared autonomy in April.

The conflict between the Southern Transitional Council (STC) and the internationally recognised government of President Abedrabbo Mansour Hadi constitutes a second front in Yemen, already split by a war between government loyalists and Iran-backed Houthis.

The STC, which declared self-rule on April 26, has made a series of military gains, the latest this month when it seized the strategic island of Socotra.

"I call on the so-called Southern Transitional Council... to return to the path of the Riyadh Agreement and stop the bloodshed," Hadi said during a meeting on Saturday with high-level government officials, referring to a power-sharing deal for the south struck last November that quickly became defunct.

The Riyadh accord's implementation "has long faltered due to continuous escalatory activities, including the announcement of self-rule and the rebellion witnessed in Socotra", Hadi said.

"Resorting to arms and force for personal gains... will not be accepted."

Hadi has lived in Riyadh since 2015, after the capital Sanaa fell to the Houthis the year before.

Located off the southern coast of Aden, the interim seat of the Yemeni government, Socotra is near strategic shipping lanes and is famed for its biodiversity.

The southern separatists and the Yemeni government are technically allies in the fight against the Houthis, but the rift between them represents a damaging "war within a war" in the Arabian Peninsula's poorest country.

Earlier this week, a Saudi-led military coalition backing the government against the Houthis said it had deployed observers to monitor a ceasefire between pro-government troops and southern separatists announced two days earlier.

Saudi forces arrived Wednesday in Shaqra and Sheikh Salem, two flashpoints in southern Yemen's Abyan province, to monitor that truce, military sources said.

A collapse of the ceasefire in the south would again complicate efforts by the government to repel the Houthis.

The separatists and the government are due to hold further talks in Saudi Arabia to discuss the truce, coalition spokesman Turki al-Maliki said this week.

Since 2015, tens of thousands of people, mostly civilians, have been killed and millions displaced in Yemen, in what the United Nations calls the world's worst humanitarian disaster.

-Agencies