The UN envoy to Yemen called on warring sides to show “restraint” and halt fighting in the central city of Marib during a nationwide ceasefire.
The two-month truce, brokered by Hans Grundberg, came into effect last Saturday and is seen as a means to pave the way to an end to the war, which the UN says has created the world's biggest humanitarian crisis, claiming nearly 250,000 lives.
“I am following very closely the latest developments in Marib and urge all parties to show restraint and their continued commitment to the truce as promised to Yemenis,” Mr Grundberg said in a statement.
The truce was announced days after the start of intra-Yemeni consultations on ending the civil war.
The talks, which ended on Thursday, were conducted in the Saudi capital Riyadh under the auspices of the Gulf Co-operation Council.
The Houthi rebels objected to the venue and refused to attend.
Yemen’s defence ministry said on Friday that forces allied with the government and tribesmen were fighting off a Houthi attack in southern Marib.
The army said it had recorded 80 breaches of the truce committed by the rebels on Wednesday as they continued fighting in Marib, Al Jawf, Saada, Hajjah, the port of Hodeidah and Taez.
For more than a year, the rebels have been attempting to advance towards the city of Marib but have been repelled by the army and the Saudi-led coalition.
The ancient city of Marib is a vital transport hub in the country's rugged north and serves as a gateway to other government-held provinces.
It is also home to a large oilfield that contains some of the country's estimated three billion barrels of reserves as well as one of the country's only refineries.
The Iran-backed Houthi rebels took over Sanaa in 2014 and ousted the internationally recognised government, prompting an intervention by the Saudi-led coalition the following year.