Fighting has raged for months in Marib, the last stronghold of the internationally recognised government.
It was ousted from power in the capital, Sanaa, in late 2014 by the Iran-aligned Houthi movement, prompting a Saudi-led coalition to intervene.
The war, which has caused the world’s worst humanitarian crisis, has been in stalemate for years. The government of President Abd-Rabbu Mansour Hadi has also fought separatists in the south who seek independence.