The U.N. envoy for Yemen says the beginning of the pullout of forces from three key ports provides an opportunity to move to the major goal of ending the four-year conflict that has created the world's worst humanitarian crisis.

Martin Griffiths told the U.N. Security Council on Tuesday that Yemen's government and Houthi Shiite rebels demonstrated that they are able to deliver on commitments they made in December by agreeing on the first phase of redeployment from the ports.

The forces could start pulling out as early as Tuesday or Wednesday.

Griffiths said forces will initially withdraw from smaller ports Salif and Ras Issa, followed by a pullout from the major port of Hodeida and critical parts of the city that will allow access to a major U.N. grain storage facility.