FILE PHOTO - Yemen's President Abd-Rabbu Mansour Hadi attends a meeting with local officials during a visit to the coutry's northern province of Marib July 10, 2016. REUTERS/ Ali Owidha

ADEN (Reuters) - Yemeni President Abd-Rabbu Mansour Hadi criticized the U.N.’s special envoy to the country in a sharply-worded letter to the U.N. chief, describing him as legitimizing Houthi rebels his Saudi-backed coalition is locked in a four-year war with.

The Iran-aligned Houthis, who ousted Hadi from power in the capital Sanaa in 2014, have stepped up missile and drone attacks on Saudi Arabia in recent days in a resurgence of tactics that had largely subsided since late last year amid United Nations-led peace efforts.

The attacks come the same month that U.N. special envoy Martin Griffiths appeared to have achieved a diplomatic breakthrough, getting the Iranian-aligned Houthis to agree a unilateral withdrawal of their forces from Hodeidah and two other ports.

Saudi Arabia and the UAE head a Western-backed coalition of Sunni Muslim states that back Hadi and intervened in Yemen in 2015 to try to restore the internationally recognized government ousted from power.

The five-page letter, addressed to U.N. Secretary-General Antonio Guterres and dated May 22, outlines a litany of grievances directed at Griffiths, criticizing “his insistence on dealing with the rebels as a de-facto government,” referring to the Houthis.

The letter states that Griffiths has failed to properly oversee an agreement struck last year in Stockholm for a ceasefire and withdrawal plan for the port city of Hodeidah, and has not dealt with issues surrounding detainees and hostages.

“It is clear the envoy has a weak understanding of the nature of Yemen’s ongoing conflict, especially the ideological, intellectual, and political elements of the Houthi militias and their fundamental rejection of the principles of democracy and the peaceful rotation of power,” stated the letter.

A UN spokesman said on Friday that Guterres reiterated his confidence in Griffiths after receiving the letter, and said the special envoy would double down on efforts to support both sides in the conflict and ensure that the Stockholm agreement is fulfilled, a U.N. statement said.