Yemen and its Saudi and UAE allies are accusing the Houthi rebels of breaking their agreement to withdraw from two ports.
Ambassadors from the three countries sent a letter to U.N. Secretary-General Antonio Guterres, asking him to demand the Houthis carry out their commitments.
"Last week's sudden and unexplained refusal by the Houthis to withdraw from the ports of Salif and Ras Issa comes as no surprise after months of stalling tactics from their side," the letter said.
The three countries who have been trying to push the Iranian-backed Houthis out of Yemen accuse the rebels of digging in by building trenches and reinforcing their military positions.
There has been no reply from the rebels, who have said it's the other side that's breaking the deal. A U.N. spokesman urges both sides to follow through with their promises.
Yemen and the Iranian-backed Houthis agreed in December to withdraw their forces from the rebel-held port city of Hodeida.
Nearly all humanitarian goods for Yemen's starving civilians come through Hodeida, and the fighting has prevented safe delivery of emergency aid.
Yemen and the Saudi-led coalition fighting the Houthis accuse them of getting Iranian weapons through the port — a charge Iran denies.