GENEVA — The United Nations on Wednesday temporarily suspended the fledgling talks aimed at ending the war in Syria and called on the countries fueling the conflict to do more to yield results, as Syrian government forces sharply escalated an offensive on a strategic rebel-held city.

“I have concluded, frankly, that after the first week of preparatory talks there is more work to be done, not only by us but by the stakeholders,” the United Nations mediator, Staffan de Mistura, said after meeting with the opposition delegation on Wednesday evening at a Geneva hotel that serves as its headquarters.

Mr. de Mistura took pains to add that the pause did not mean “the end or the failure of the talks.” In a statement later in the evening, he suggested that the government’s failure to alleviate the humanitarian crisis in Syria by allowing food and medicine into rebel-held towns had prevented any serious discussions.

“I’m not prepared to have talks for the sake of talks,” he said, adding that they would resume no later than Feb. 25.