Fierce fighting raged Saturday in south Yemen between Iran-backed rebels and loyalists of exiled President Abd Rabbu Mansour Hadi, as pressure mounted for the warring factions to hold political talks.
The Houthi rebels, who have overrun large parts of the country and forced Hadi to flee overseas, have demanded a complete end to a month of Saudi-led air strikes against them as a condition for UN-sponsored talks.
Clashes left at least 38 people dead on Saturday in towns in the south of the impoverished country, strategically located next to oil-rich Saudi Arabia and key shipping routes.
Former strongman Ali Abdullah Saleh, who still holds sway over army units allied with the Shiite rebels, late on Friday urged the Houthis to heed UN demands to withdraw from territory they have seized.
US Secretary of State John Kerry also called on the anti-government forces to enter into political dialogue to end a conflict that the UN says has killed more than 1,000 people since late March.
"This has to be a two-way street," Kerry told reporters, adding: "We need the Houthis and we need those that can influence them to make sure that they are prepared to try to move... to the negotiating table."
UN Secretary General Ban Ki-moon has announced plans to appoint Mauritanian diplomat Ismail Ould Cheikh Ahmed as his new envoy to Yemen after the previous pointman lost the confidence of Gulf countries.
The appointment becomes official on Monday if no objections are raised by the 15-member council.
The Houthis swept into Sanaa in September from their northern stronghold and then advanced south on the major port of Aden, forcing Hadi to flee to Riyadh last month.
The capital remains in their hands while Al-Qaeda has exploited the instability to seize more territory in the largely lawless southeast.
The fighting has raised fears that Yemen could become a new front in a proxy war between Sunni-ruled Gulf monarchies and Shiite Iran.
Tehran rejects accusations it armed the rebels and has presented a peace plan to the UN calling for a ceasefire and the formation of a unity government.
A US aircraft carrier headed to Yemeni waters earlier this week to monitor an Iranian convoy that had raised suspicions but redeployed on Friday after the convoy turned back, Pentagon officials said.
At least seven Hadi loyalists and 22 Houthis were killed in dawn fighting Saturday in the town of Daleh, north of Aden, an official said.
Farther east, in Loder, loyalist militiamen killed nine rebels in a rocket-propelled grenade attack, a government official in the town said.
There were also heavy clashes in Aden, where the coalition kept up air strikes days after announcing its campaign was entering a new phase aimed at resuming the political process, delivering aid and fighting "terrorism".
Targets included the rebel-held presidential palace, which was Hadi's last refuge before he fled to neighbouring Saudi Arabia last month, military officials said.
Coalition warplanes also bombed the rebel-held Al-Anad air base north of Aden, which housed US troops supporting a long-running drone war against Al-Qaeda before the fighting forced their withdrawal.
There was also fighting late on Friday in the eastern province of Marib, home to some of Yemen's most important oil fields, army officers and witnesses said.
Loyalist troops at a base in the provincial capital shelled rebel positions in the nearby Sarwah district, where clashes raged around Yemen's main oil export pipeline.
The 435-kilometre (270-mile) line links Marib's Safir oil fields with the Ras Isa terminal on Yemen's Red Sea coast and control of it has been a key goal for the rebels and their allies.
The UN says millions have been affected by the conflict and are struggling to access healthcare, water, food and fuel.
It estimates that at least 551 of those killed since late March were civilians and included at least 115 children.
Amnesty International called for an urgent investigation into the civilian deaths.
"Some of the Saudi Arabian-led air strikes appear to have failed to take necessary precautions to minimise harm to civilians and damage to civilian objects," said its Middle East and North Africa deputy head, Said Boumedouha.
"It is crucial that independent and impartial investigations are carried out to ascertain whether violations of international humanitarian law have been committed."