Saudi-led coalition jets have bombed Houthi rebels in Yemen’s third city of Taiz, hours after announcing the end of a military campaign against them.
The strikes followed the fall of the base outside Taiz of an army unit loyal to President Abdrabbuh Mansour Hadi.
Fighting was also reported in the second city of Aden, Lahj’s provincial capital Huta, and the town of Daleh.
On Tuesday, Riyadh declared its month-long campaign, which sought to restore the president, had achieved its goals.
But it warned that it would continue to take action against the Houthis as needed.
The UN says at least 944 people have been killed and 3,487 injured in air strikes, fighting on the ground and attacks by jihadist militants in Yemen since 19 March.
Not long after dawn on Wednesday, rebel fighters took control of the headquarters of the 35th Armoured Brigade on the northern outskirts of Taiz.
An army officer told the AFP news agency that dozens of people were killed or wounded in the heavy fighting, and that shortly after the takeover coalition aircraft targeted rebel forces inside the base.
They also bombed a gathering of rebels and allied military personnel loyal to ousted former President Ali Abdullah Saleh near the city’s central prison, he added.
The 35th Brigade has stayed loyal to President Hadi, who fled the country at the end of March as the Houthis and their allies advanced on Aden.
Analysis: Frank Gardner, BBC security correspondent
Yemen is embroiled in a messy and costly conflict with fluid battle lines and shifting loyalties. So any peace deal to end it is unlikely to translate into immediate calm on the ground.
The Saudis, who lead the multinational coalition of air forces bombing the Houthi rebels and their allies, had clear war aims from the outset. These were: make the Houthis withdraw from all the ground they have seized and reinstate their ally, the ousted President Hadi.
These demands and others were supported by the UN Security Council and the Saudis say the Houthis have agreed to most of them.
But Yemen’s power-sharing problems are multiple and chronic. Mr Saleh has the potential to wreck any deal, the Southerners around Aden now want independence from the North more than ever, and in the East al-Qaeda has profited from the chaos by grabbing as much territory and weapons as they can.
The president took refuge in the southern port city of Aden after the rebels took full control of the capital Sanaa and placed him under house arrest.
Saudi diplomatic sources told BBC Middle East editor Jeremy Bowen, who is in Jeddah, that the air strikes had resumed because the Houthis “broke a promise” in Taiz.
The Saudis and their coalition partners reserved the right to use air power again when they announced the end of “Operation Decisive Storm” on Tuesday.
“There might be less frequency and the scope of the action might be less, but there will be military action,” spokesman Brig Gen Assiri told a news conference in Riyadh.
A new, more limited campaign, “Operation Restoring Hope”, would focus on preventing the rebels from “targeting civilians or changing realities on the ground” and finding a political solution the conflict, Gen Assiri said.
The Saudi diplomatic sources said an agreement was being finalised with the Houthis. If the deal is made, making it stick will be hard, our correspondent adds.
In an apparent gesture of goodwill on Wednesday, the Houthis released Defence Minister Mahmoud al-Subaih, the brother of President Hadi and a military commander, rebel and tribal sources said. The three men had been held for nearly a month.
Former President Saleh also urged all sides to “return to dialogue to solve and treat all the issues”, in a message posted on social media.
Iran, which has denied that is providing military assistance to the rebels, described the Saudi decision as a positive development and called for peace talks.
But Pro-Hadi militiamen in southern Yemen declared that they would continue fighting the rebels until they had driven them out of the region.
“The end of Operation Decisive Storm does not mean that the southern resistance will end its operations on the ground,” a statement said. “This front will not stop its fight until all the South is purified from the Houthis and pro-Saleh forces.”
There were exchanges of tank fire between the militiamen and Houthi fighters in Aden on Wednesday, residents told the New York Times.