CAIRO — Yemen's warring sides Sunday resumed United Nations-backed negotiations over a prisoner swap, the world body said, more than three months after they completed the war's largest exchange.

The talks in the Jordanian capital Amman between representatives of the internationally recognized government and the Houthi rebels, came less than a week after the U.S. designated the Iranian-backed rebels as a terrorist group.

The U.N.'s special envoy for Yemen, Martin Griffiths, urged the gathering in Amman to prioritize "the immediate and unconditional release of all sick, wounded, elderly and children detainees as well as all arbitrarily detained civilians, including women."

The talks in Amman are facilitated by the U.N. and the International Committee of the Red Cross, Griffiths' office 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 a peace deal struck in 2018 in Sweden.

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 meeting in Amman came after the U.S. declared the Houthis a "foreign terrorist organization," a designation that took effect Jan. 19, a day before Joe Biden was inaugurated president.

The U.S. move prompted the U.N. secretary-general and other humanitarian officials to urge Washington to reverse the designation to prevent massive famine and death in Yemen.

Twenty-two aid groups working on Yemen Sunday called for the designation of Houthis as a terrorist organization to be "revoked immediately," warning about "potentially catastrophic humanitarian impact."

The groups, including Norwegian Refugee Council, Oxfam, Save the Children and CARE International, argued that the humanitarian exemptions the U.S. has made will not shield the war-torn Yemen from "another major economic shock at a time when people are struggling to make ends meet."

They warned about possible "delays and uncertainty" that would hinder their operations in Yemen, particularly in areas held by the Houthis, where most of the people in need live.

"Any disruption to lifesaving aid operations and commercial imports of food, fuel, medicine and other essential goods will put millions of lives at risk," the groups said in a joint statement.

Yemen is in the world's worst humanitarian crisis. The war that has killed more than 112,000 people and wrecked the country's roads, hospitals, water and electricity networks, as well as other infrastructure.