Marib, which lies around 120 km (75 miles) east of the capital Sanaa, had been the main frontline before the truce was first agreed last April, as forces of a Saudi-led military coalition repelled advances by the Houthi movement bent on seizing full control of the province in central Yemen.
Yemen has been mired in violence since the Houthis ousted
the government from Sanaa in late 2014, prompting the Saudi-led
coalition to intervene months later.
Marib is the last stronghold in northern Yemen of the
internationally recognised government and is the country's sole
gas producing region with a large oilfield.
The escalation comes at a time Saudi Arabia and the Houthi
movement have been holding direct talks in parallel with United
Nations efforts to reinstate and expand the truce deal.
U.N. special envoy Hans Grundberg earlier this week voiced
hope things were moving "in the right direction", citing recent
developments including a prisoner swap between the warring sides
and an agreement between Saudi Arabia and Iran to restore
bilateral ties.
The conflict is widely seen as a proxy war between Saudi
Arabia and Iran. It has killed tens of thousands of people,
wrecked Yemen's economy and pushed millions into hunger.
(Reporting by Mohammed Alghobari and Reyam Mukhashaf; Writing
by Lisa Barrington; Editing by Raissa Kasolowsky)