The US on Tuesday urged Yemen's Iran-backed Houthi rebels to stop their offensive on the government-held city of Marib and take part in international efforts to find a political solution.
"The United States urges the Houthis to halt their advance on Marib and cease all military operations and turn to negotiations," the State Department said.
"The Houthis’ assault on Marib is the action of a group not committed to peace or to ending the war afflicting the people of Yemen."
The offensive on Marib city, a stronghold of the internationally recognised government, threatens to complicate a renewed diplomatic push to end the war.
Marib has been a refuge for hundreds of thousands of people fleeing violence during Yemen's six-year war.
US special envoy for Yemen Tim Lenderking said the US was "aggressively" using back channels to communicate with the group.
"We do have ways of getting messages to the Houthis and we are using those channels very aggressively as we're engaging ... in person with the leadership of the key countries involved," Mr Lenderking told a State Department briefing on Tuesday.
The recent push towards Marib by Houthi forces comes alongside intensified drone attacks into Saudi Arabia by the group.
The UN and the new Biden administration have sought to start talks to end the war.
The conflict has created the world's largest humanitarian crisis, the UN has said.
Mr Biden took the first step towards diplomacy by revoking the previous Trump administration's terrorist designation of the Houthis.
Despite removing the terrorist designation from the Houthis, US Secretary of State Antony Blinken said last week that rebel leaders remained under US sanctions.
Mr Blinken said the Biden administration was “actively identifying additional targets for designation, especially those responsible for explosive boat attacks against commercial shipping in the Red Sea” and drone and missile attacks on Saudi Arabia.
The UN special envoy to Yemen, Martin Griffiths, has said the resumption of Houthi hostilities near Marib was extremely concerning at a time of renewed diplomatic momentum.
Yemen's front lines have largely been in stalemate for years but a major Houthi gain in Marib would leave the group in control of what historically was known as North Yemen.
Marib city is also the last line of defence before Yemen's biggest gas and oilfields, which are in government hands.
The UN's International Organisation for Migration says 106,449 people have been displaced by fighting along the Marib front lines over a year.
It said another 385,000 people could be displaced with a major front-line shift. There are 125 displacement sites around Marib, the UN agency says.