Fighting has broken out in the Yemeni city of Hudaydah despite both the Saudi-led military coalition and Houthi rebels agreeing to halt offensives.
Coalition warplanes resumed bombing following clashes between rebels and pro-government forces.
The UN is attempting to revive talks to end a three-year war that has caused a massive humanitarian crisis.
So far, the conflict has killed thousands and pushed millions more Yemenis to the brink of starvation.
On Monday, intense fighting could be heard on the outskirts of the city, which is controlled by the Houthi rebels, while coalition warplanes launched a series of air strikes on strategic rebel positions, AFP news agency reports.
What had each side recently agreed?
The head of the Houthis' Supreme Revolutionary Committee, Mohammed Ali al-Houthi, earlier said in a statement that the rebels were halting drone and missile strikes on coalition forces after a request from the United Nations.
The statement added that Houthis were ready to move towards a wider ceasefire if "the Saudi-led coalition wants peace".
Last week the coalition, which is led by Saudi Arabia and the United Arab Emirates, ordered a halt in its offensive on Hudaydah, a port on the Red Sea. They have also said that they support the UN-led talks.
The UN special envoy to Yemen, Martin Griffiths, has said he hopes peace talks convene before the end of the year.
He told the UN Security Council on Friday that he had been given "firm assurances" that both sides would attend, promising to accompany the Houthi representatives from the Yemeni capital Sanaa if need be.
The Houthis failed to show up at peace talks in September.
Why is this battle important?
The latest violence in Hudaydah is likely to set back international efforts to bring an end to the ongoing conflict.
Hudaydah's port is a key lifeline for just under two-thirds of Yemen's population, which is almost totally reliant on imports of food, fuel and medicine.
The UN has previously warned that in a worst-case scenario, the conflict could cost up to 250,000 lives, as well as cut off aid supplies to millions of people.
What is behind the conflict?
The conflict began in 2014 when the Houthi Shia Muslim rebel movement took advantage of the new president's weakness and seized control of northern Saada province and neighbouring areas.
The Houthis went on to take Sanaa, forcing President Abdrabbuh Mansour Hadi into exile abroad.
The conflict escalated dramatically in March 2015, when Saudi Arabia and eight other mostly Sunni Muslim Arab states - backed by the US, UK, and France - began air strikes against the Houthis, with the declared aim of restoring Mr Hadi's government.