RIMBO, Sweden (AP) — Both sides in Yemen's civil war agreed Tuesday to exchange more than 15,000 prisoners by Jan. 20, a member of the rebel delegation said, in what could be the first major breakthrough in the 4-year-old conflict that has killed thousands of people and left millions more facing hunger.

Delegations from the internationally recognized government of President Abed Rabbo Mansour Hadi and Iran-backed Shiite rebels said at news conferences in Sweden that they have exchanged prisoner lists. The lists will be reviewed over four weeks, ahead of a final swap to be facilitated by the International Committee of the Red Cross, the representatives said.

The move is a crucial step in implementing an agreement reached earlier this month on the exchange of all prisoners held by both sides.

The government, which is backed by a Saudi-led coalition, said it has provided U.N. mediators with an initial list of 8,200 prisoners allegedly held by the rebels, known as Houthis.

The government list included members of the family of former President Ali Abdullah Saleh, who was killed by the Houthis in December 2017. It also included more than 300 children and 88 women.

The rebel delegation said it has provided its own list of prisoners held by the government.

"The Houthis provided a list of 7,487 captives and detainees. We provided a (list) of 8,576 detainees," said Askar Zouail of the government delegation. "But the number of detainees (held by the rebels) exceeds 18,000 detainees since the beginning of the war" in March 2015.

While U.N. officials at the talks called the exchange of prisoner lists "very impressive," the sheer size of the swap could potentially delay the process beyond the Jan. 20 deadline and hinder implementation of the deal.

Othman Mujali, who is agriculture minister in the Hadi government, said both sides are considering a mechanism for implementing the swap.

"We hope the other side is serious. For us, we are serious and ready at the moment," he said.

Houthi representative Abdul-Qader el-Murtaza said the government list includes prisoners from member countries in the coalition, such as Sudan and the United Arab Emirates.

He said Jan. 20 was set a final date for the prisoner swap overseen by the Red Cross.

A period of four weeks was set for any questions and for verification of the prisoners' names, "then there will be 10 days for the Red Cross to prepare logistic and technical matters for the transfer and exchange of the prisoners," el-Murtaza said.

The U.N. is hopeful that details of the prisoner release will be worked out and "we can handle the logistics" with partners, said U.N. deputy spokesman Farhan Haq.

"If this can be achieved, that certainly is a positive step forward," he told reporters at U.N. headquarters in New York. "Any measures that involve increasing the amount of trust and confidence that's built up can, we believe, help achieve a lasting resolution to this conflict."

Martin Griffiths, the U.N.'s Yemen envoy, told reporters in Sweden on Monday that he was encouraged by "the positive and serious spirit" both sides have demonstrated in the current round of talks.

The Shiite rebels seized the capital of Sanaa in 2014. The Saudis intervened the following year. The civil war has killed at least 10,000 people, though the figure is believed to be higher, and turned Yemen into one of world's worst humanitarian crises with 22 million of its 29 million people in need of aid, according to the U.N.

While the prisoner agreement created a measure of goodwill between the warring sides, an agreement to reduce violence in the strategic port city of Hodeida would create a climate more conducive toward seeking a settlement to the conflict.

Rebel delegation member Gamal Amer said both sides are discussing initial draft proposals for Hodeida. The proposals include a halt to all fighting, the withdrawal of troops to the city limits and later out of the province, while allowing U.N. oversight and setting up a local administration, Amer said in video comments.

An agreement on Hodeida is expected to be reached before heading to another future round of peace talks, Amer added, without elaborating.

About 70 percent of food aid and other imports are shipped through the port of Hodeida, and reducing restrictions on aid organizations and fuel imports are vitally needed.

U.N. humanitarian chief Mark Lowcock said Monday that 20 million people, amounting to 70 percent of the population of Yemen, are hungry, adding that there has been "a significant, dramatic deterioration" of the humanitarian situation in the country.

Saudi Ambassador to Yemen Mohammed al-Jaber lauded the "positive atmosphere at the talks.

"We are optimistic progress can help with restoration of Yemeni state, security, and stability," he tweeted on Tuesday.

Iranian Foreign Minister Mohammad Javad Zarif also welcomed the initial agreements between the Yemeni parties in Sweden under U.N. supervision.

He tweeted that Iran "strongly supports continuation of talks to achieve final accords on all issues. It is well past time for foreign aggressors to end their airstrikes & crimes against humanity." He was referring to the coalition, which is supported by the United States.

U.N. officials have sought to manage expectations in the talks, saying they don't foresee rapid progress toward a political settlement but hope for at least some minor steps that would help the worsening humanitarian crisis and prepare a framework for further negotiations.

An Associated Press investigation has revealed evidence of torture at detention sites run by the Houthi rebels. There has also been international outrage over abuses by the coalition. The AP has previously exposed torture at secret prisons run by the UAE, a coalition member, and Yemeni allies. It has documented the deaths of civilians from drone strikes by the U.S. in its campaign against al-Qaida's branch in Yemen.

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Magdy reported from Cairo. Associated Press writer Edith M. Lederer at the United Nations contributed.

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