CAIRO — Houthi rebels in Yemen said they had started to withdraw from three strategic Red Sea ports on Saturday, offering a tentative boost to faltering United Nations-led efforts to start talks on ending the four-year war in the country.

Mohammed Ali al-Houthi, the head of the rebels’ Supreme Revolutionary Committee, said they had started to pull out of Hudaydah, the main conduit for humanitarian aid into Yemen, and two smaller nearby ports, Salif and Ras Isa.

The extent of the withdrawal was unclear. United Nations officials posted to Yemen had not offered any confirmation by Saturday afternoon. The Saudi-led coalition fighting the Houthis, whose troops are stationed on the southern and eastern edge of Hudaydah, offered no comment.

The United Nations envoy to Yemen, Martin Griffiths, described the process as a “fragile vessel.” Many worry that its failure could plunge Yemen into a new, even more destabilizing, round of fighting.