The U.N. envoy for Yemen is urging the country's warring parties to maintain the momentum of the initial pullout of Houthi rebel forces from three port areas.

Martin Griffiths is urging the sides to move quickly to larger joint withdrawals — and to work urgently on a political solution to the devastating war.

Griffiths told the Security Council on Wednesday there are "signs of hope" but also "alarming signs in recent days" that progress can be threatened.

The war erupted in 2014 when the Houthis seized the capital, Sanaa. A Saudi-led coalition joined the war the following year on the side of the government.

Despite the significance of the recent Houthi withdrawal from the key port of Hodeida and two smaller ports, he says: "Yemen remains at the crossroads between war and peace."