CAIRO (AP) — Yemen’s warring sides did not achieve progress in their latest round of negotiations over a prisoner swap, the United Nations said on Sunday.

The talks started late last month in the Jordanian capital Amman between representatives of the internationally recognized government and the Houthi rebels. The prisoner exchange is part of a peace deal struck in 2018 in Sweden.

U.N. Special Envoy for Yemen Martin Griffiths said the two sides did not agree to release prisoners during this round of talks concluded Sunday in Amman.

“I am disappointed that this round of talks did not amount to what we saw in Switzerland last September which resulted in the historic release of 1056 detainees,” he said.

In October, the warring sides achieved the largest-ever prisoner swap of the war, releasing more than 1,000 detainees. That followed occasional releases of dozens of prisoners over the past two years, which also served as gestures of good faith, stoking hopes the factions would implement the 2018 agreement.

The prisoner swap talks were facilitated by the U.N. and the International Committee of the Red Cross.

Griffiths urged the warring sides to work on “the implementation of what they agreed to and expand the arrangements to release more detainees soon.”

He repeated his calls for “the unconditional release of all sick, wounded, elderly and children detainees as well as detained civilians, including women and journalists.”

The two sides traded the blame for the failure of the talks.

Yemen’s devastating conflict erupted in 2014, when the Houthis seized the capital, Sanaa, and much of the country’s north. That prompted a U.S.-backed Arab military coalition to intervene months later in a bid to restore the government of Yemeni President Abed Rabu Mansour Hadi to power.

The conflict has killed some 130,000 people and spawned the world’s worst humanitarian disaster.

The failure in the prisoner swap talks has come amid an intensive attack by the Houthis on the government-held Marib province.

The Houthi attacks forced several thousand of internally displaced people, or IDPs, to flee from Marib’s district of Swarih eastward to the province’s capital, after the heavy fighting left them without water, electricity, health and educational needs.

Marib province has served as a sort of haven for around 1 million Yemenis who have fled Houthi offensives since the start of the war.