Houthi military forces are intensifying their push towards the central Yemeni city of Marib, which is held by the Saudi-backed government, and are stepping up fighting in the south, Houthi group and Yemeni military sources said on Thursday.

After recent advances and fierce fighting, Houthi military spokesman Yahya Sarea said the group's fighters were on the western outskirts of Marib city and pushing up on other fronts having inflicted many casualties in recent months.

A Yemeni government military source said Houthi forces are around 18 km west of Marib city, but the main fighting has been in the southern region of Shabwa, which has several oil fields and the country's sole liquified natural gas terminal.

Houthi forces are advancing into the Assilan district in Shabwa, where the Janna oil field is located.

Marib lies about 120 km (75 miles) east of the capital Sanaa, which the Iran-aligned Houthis seized along with most of north Yemen in 2014 when they ousted the internationally recognised government of President Abd-Rabbu Mansour Hadi.

They have since early this year advanced on three fronts towards the Marib region, which is the government's last northern stronghold and has Yemen's biggest gas fields. There have been many casualties on both sides.

Saudi Arabia and the United Arab Emirates intervened in the war in 2015 to try to restore Hadi's government to power but the conflict has dragged on, killing tens of thousands and causing the world's worst humanitarian crisis.

The escalation in fighting comes as U.N. and U.S. envoys have been in the region to try to revive stalled peace talks.

Reuters