CAIRO (AP) — Yemen’s warring sides quietly began U.N.-backed talks about a prisoner swap on Monday, according to the United Nations.

U.N. deputy spokesman Farhan Haq said a third round of negotiations aimed at implementing an ambitious Yemeni prisoner exchange was underway in the Jordanian capital of Amman.

Although heavy clashes continue on the ground, the resumption of talks signals modest headway in diplomatic efforts to resolve a war that has killed over 100,000 people and pushed millions to the brink of famine.

The prisoner swap deal was seen as a breakthrough during 2018 peace talks in Sweden, where Yemen’s Shiite Houthi rebels and the internationally recognized government agreed to several such confidence-building measures, including a cease-fire in the strategic port city of Hodeida.

Ongoing military offensives and deep-seated distrust have dogged implementation of the tentative peace plan. But more recently, hopes for a settlement have been stirred by back-channel talks in the Gulf state of Oman between Saudi Arabia, which supports the government of President Abed Rabbo Mansour Hadi, and the rebels.

The talks focus on interim agreements, such as re-opening Yemen’s main international airport in Sanaa, which was shut down by the Saudi-led coalition in 2016. In a sign of progress, two United Nations flights ferrying dozens of seriously ill Yemenis abroad for treatment took off last week from the rebel-held capital, the first since the start of the air blockade.

The conflict in the Arab world’s poorest country erupted in 2014, when the Iran-allied Houthis seized the capital and much of the country’s north. A Saudi-led coalition, determined to restore Hadi’s government, launched a military intervention months later. Yemen's worsening humanitarian catastrophe has added a sense of international urgency to political negotiations.