SANAA, YEMEN -- The UN special envoy for Yemen on Monday expressed frustration that his efforts to achieve a cease-fire in the war-torn country have been derailed by warring parties seeking gains on the battlefield.

Martin Griffiths said ongoing fighting in several parts of Yemen, including a monthslong attack by Houthi rebels on government-held Marib province, has undermined the prospects for peace in the country.

Yemen has been embroiled in a civil war since 2014, when the Iranian-backed Houthis swept across much of the north and seized the capital, Sanaa, forcing the internationally recognized government into exile.

A Saudi-led coalition entered the war the following year on the side of the government. The war has killed more than 130,000 people and spawned the world's worst humanitarian crisis.

“Nobody can be more frustrated than I am,†Griffiths said. “We have spent a year and a half on things which are relatively simple to describe, the cease-fire, the opening of Sanaa Airport, the opening of Hodeida ports, the much-delayed start of the political negotiations.â€

He spoke Monday in a news conference at the Sanaa airport, a day after a video meeting with the Houthis' religious and military leader, Abdel-Malek al-Houthi, who was in an undisclosed location.

Griffiths also held talks with Yemeni and Saudi officials in Saudi Arabia and Oman, part of his efforts to achieve a cease-fire, reopen the Sanaa airport, ensure an uninterrupted flow of fuel and commodities through Hodeida ports, and relaunch the political process.

“We have been negotiating this in detail. ... Sometimes we make good progress, and we think that it's going to work, that we will get an agreement. And then the war intervenes and one or other party thinks they will gain more in the battlefield,†he said.

He urged the sides to seize the “considerable regional and international support†for the UN peace plan to “bring this negotiation, this long negotiation to a successful conclusion.â€

UN Secretary-General Antonio Guterres announced earlier this month the appointment of Griffiths as the UN's next humanitarian chief. But Guterres said Griffiths will continue to serve as the UN's top envoy for Yemen “until a transition has been announced.â€