At least 26 fighters were killed in clashes on Monday between pro-government troops and rebels over control of an oil-rich region east of the Yemeni capital, military sources said.
"Pro-government forces launched a military operation today to retake Sarwah," a loyalist military source said of the only part of Marib province still held by the rebels who have controlled nearby Sanaa since September 2014.
Clashes and air raids by the Saudi-led coalition backing Yemen's internationally recognised government "killed 16 rebels and left dozens wounded", the source said.
Ten pro-government soldiers were killed and 12 wounded in the fighting, he added.
Loyalist forces recaptured hills overlooking Sarwah, military sources said.
Marib province has seen fierce battles between forces loyal to President Abedrabbo Mansour Hadi and Shia Houthi rebels and their allies.
If they controlled Marib, loyalist forces could advance from the east towards Sanaa.
A Saudi-led coalition which intervened in March last year against the Iran-backed rebels has forces based in Marib to support the loyalists.
It has intensified operations since the suspension in early August of UN-brokered peace talks.
More than 6,600 people have been killed in the Yemeni conflict since March 2015, according to the United Nations.