The Saudi-led coalition fighting in Yemen said seven Houthi prisoners will be released after a Saudi soldier was freed and arrived in Riyadh in a rare exchange of goodwill between combatants in the devastating three-year war.

Saudi prisoner Mousa Awaji returned on a Red Cross plane from Sanaa on Tuesday with the rebels saying he was freed because of an illness, Houthi TV channel al-Masirah reported, citing Abdulqadir Murtada, a Houthi official.

Coalition spokesman Turki al-Maliki said the exchange came because the Houthis did not provide Awaji with proper medical treatment and efforts were under way to end the detention of other prisoners, state media reported.

Murtada said the release came as an "initiative" by Houthi leader Abdul-Malik al-Houthi, according to the Houthi-run Saba news agency.

The United Nations special envoy to Yemen, Martin Griffiths, helped coordinate the release. In a tweet, he welcomed the gesture and said he looked forward to the implementation of a prisoner-exchange agreement.

The SE welcomes the unconditional release by AA of the sick Saudi prisoner, whom the ICRC will transfer from Sanaa to Riyadh today. The SE hopes to see more similar humanitarian gestures from the parties. #Yemen — OSE_Yemen (@OSE_Yemen) January 29, 2019

Afrah Nasser, a Yemen political analyst, told Al Jazeera it was important for the international community to pressure both sides to release more prisoners of war. She added the rebels' move also had symbolism.

"It's very remarkable that they released a sick Saudi captive in order to send a message about the destroyed or devastated healthcare system. The Houthis are really playing it clever at this moment," Nasser said.

Yemen's warring parties have yet to agree to full terms of a prisoner swap - one of the least contentious confidence-building measures agreed at UN-sponsored peace talks held in December amid Western pressure to end the bloody conflict.

The UN is pushing for the exchange and the implementation of a ceasefire in the main port city of Hodeidah to pave the way for a second round of discussions to end a war in which tens of thousands of people have been killed in almost four years.

Agreement in peril

While the prisoner swap was a positive sign, a humanitarian group warned on Tuesday that a ceasefire agreed in Hodeidah is on the verge of collapsing, after a retired Dutch general in charge of the truce stepped down from his role.

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The US-based International Rescue Committee said recent clashes in the city between Houthi rebels who control it and pro-government forces backed by a Saudi-led coalition have increased dramatically since last week.

"In recent days, with clashes erupting inside Hodeidah and both parties accusing each other of violations, the agreement is increasingly in peril," Frank McManus of the group said.

The developments threaten to unravel a ceasefire and prisoner swap signed in December, the group said, urging the international community to step up pressure on the warring parties to stick to their commitments.

A demining team came under fire while trying to clear access to grain silos in Hodeidah on Tuesday.

The Houthis said one of its members died when Saudi-led forces fired on a demining team. The internationally recognised Yemeni government said Houthis attacked a UN-backed team heading to the mills.

The World Food Programme (WFP) has since September been unable to access the Red Sea Mills, where 51,000 tonnes of UN wheat is stored - enough to feed 3.7 million people for a month.

UN officials were not immediately available to comment, while WFP only said it was aware of the reports.

The grain facility is at a front-line flashpoint on the eastern outskirts of the city. Last week, two silos were damaged by fire caused by suspected mortar shelling.

The Houthis said the incident was a violation of the UN-sponsored ceasefire for Hodeidah.

"The other side's audacity has reached the point of targeting an operation to clear the road and in the presence of the UN team overseeing it," the rebels said in a statement on Houthi-run media.