Fresh fighting has been reported in the southern Yemeni city of Aden between Houthi rebels and militiamen loyal to President Abdrabbuh Mansour Hadi.
Several houses in the central Crater district were set on fire after being hit by rockets as the rebels advanced, residents told the Reuters news agency.
Warplanes of the Saudi-led coalition, which backs the government, meanwhile bombed rebel targets to the north.
Iran is also reported to have sent navy vessels to the Gulf of Aden.
Southern Yemeni militias backed by warplanes from a Saudi-led coalition attacked Houthi fighters across several provinces in south Yemen on Monday, driving the Shi'ite rebel forces from some of their positions, witnesses and militia sources said.
The southern fighters' gains came on the 12th day of an air campaign by Saudi Arabia and mainly Gulf Arab allies trying to stem advances by the Iran-allied Houthis, who control the capital Sanaa and have advanced on the southern city of Aden.
The fighting has killed hundreds of people, cut off water and electricity supplies and led the United Nations children's agency UNICEF to warn that Yemen is heading towards a humanitarian disaster.
Saudi Arabia, the main Sunni Arab power in the Gulf, launched the air campaign on March 26 to try to contain the Shi'ite Houthis and restore President Abd-Rabbu Mansour Hadi, who has fled Aden for refuge in Riyadh.
The International Committee of the Red Cross and UNICEF plan to fly aid planes into Yemen on Tuesday, but the missions have been delayed as they seek clearance from Arab states waging the air strikes and hunt for planes prepared to fly to Yemen.
In Aden, Houthi forces gathered at the edge of the main port area on Monday but pulled out of two residential quarters on its fringes, residents told Reuters. Around 60 people were killed in heavy fighting in the area on Sunday, they said.
Explosions shook Aden's suburbs as residents reported a foreign warship shelling Houthi positions on the outskirts.
Military momentum is hard to judge in a disjointed conflict playing out across hundreds of miles of mountains, deserts and coastal positions, but in the southern provinces surrounding Aden the Houthis' foes said they had made gains.
