Iran-backed Houthi rebels are sending reinforcements to the western outskirts of Marib, the last stronghold of Yemen's internationally recognised government in the north, local tribes told The National.
Marib, several kilometres from an ancient city of the same name, has been the centre of a grinding war of attrition.
Heavy air strikes from the Saudi Arabia-led coalition have struck Houthi positions in the surrounding mountains.
Local tribes loyal to the government have been crucial in holding the city.
"New Houthi reinforcements arrived from the provinces of Sanaa and Amran to boost their fighters following heavy losses after fierce clashes with our troops in Al Mashjah and Al Kassara frontlines," Sheikh Mohammed Al Qardaie, a tribal leader in southern Marib, told The National on Friday.
The latest round of fighting follows a plea from the UN's Yemen envoy, Martin Griffiths, calling for a stronger international effort to end the conflict.
Marib had become the “major centre of gravity” in the war, Mr Griffiths told the UN Security Council on Friday.
"The Houthis have been desperately pushing their troops forward, attempting to cut the supply lines of our troops in Al Mashjah, west of the city, using long-range missiles and drones, but they haven't scored any significant progress " Sheikh Mohammed said.
The new escalation comes after fierce confrontations that have turned the battle for Marib into a war of attrition, with mounting losses for both sides.
"More than 57 of the Houthi fighters were killed amid heavy clashes in Al Kassara and Al Mashjah front in last couple of days," a pro-government military source in Marib city told The National.
"Last Thursday, clashes in Al Mashjah fiercely erupted at night and continued until the morning.".
The source said the Houthi rebels took control of a military base controlled by pro-government troops in Al Mashjah, but pro-government forces recaptured the site on the same day.
In southern Marib, Houthi militias thwarted an attack by fighters from the Murad tribes on Thursday night on Al Ahmed mountain.
"Three Houthi fighters were injured and one was captured amid the attack, but our fighters withdrew," a tribal source told The National.
The Houthis are pressing their offensive towards the city centre to gain more leverage in UN-brokered peace talks, which are supported by Washington.
The Houthi escalation around Marib has posed a threat to the city, which hosts more than a million internally displaced people.
Some displaced families have had to be evacuated from camps and makeshift dwellings twice since February, as the Houthis closed in and, in some cases, shelled them.
Some families living in the camps west of the city have been suffering harsh conditions amid severe shortages of clean water and power supplies.
"More than 1,800 familes live in Al Swaida camp in northern Marib, lacking clean water and power in the holy month when people need the cold water the most," Dr Khaled Musaed, the manager of the government's department for displaced people, said on Friday.
