A ceasefire between Yemen's Houthi group and the Arab coalition is in danger of collapse, with each side accusing the other of violating it.
The threat of collapse appeared just hours after a mass local-level prisoner exchange on Wednesday, coinciding with UN-sponsored talks in Switzerland.
Brigadier General Ahmed al-Assiri, of the Arab coalition assembled by Saudi Arabia, said the Houthis had repeatedly broken the ceasefire and that his forces were responding.
The truce was intended to last for seven days and coincide with the peace talks to try to end a nine-month-old civil war between the Houthis based in Yemen's north and the coalition-backed southern and eastern fighters loyal to President Abd-Rabbu Mansour Hadi.
"The number of violations is around 150 and this does not show honest intentions," Assiri told Al Ekhbariya television.
"We urge the United Nations to clarify to the Houthis that there will be no patience towards these practices and the truce could collapse at any moment."
The remarks came after a spokesman for Yemeni government forces loyal to former President Ali Abdullah Saleh, an ally of the Houthis, accused the Arab coalition of a "serious escalation by land, sea and air", according to the Houthi-controlled Saba news agency.
"We will respond strongly towards the breaches that are taking place by the alliance and their mercenaries," he was quoted as saying by the Houthi-run Saba news agency.
Meanwhile, expectations for a deal at the peace talks in the Swiss city of Biel are low.
The swap - that was negotiated at a local level in coordination with the Yemeni tribes and did not involve international efforts - was taking place a day after the truce came into force.
Later on Wednesday, Yemeni officials told Al Jazeera in Switzerland that there was a failed attempt at negotiating another swap.
The second exchange would have involved the release of Yemeni government officials, notably Mahmoud al-Subaihi, Hadi's defence minister, but the Houthis demanded a full ceasefire and an end to all hostilities first.
The government, on the other hand, wanted the prisoners released before anything else.
'Path of peace'
Earlier on Wednesday, Khaled al-Yamani, Yemen's ambassador to the UN, told Al Jazeera that government forces would respect the ceasefire even in case of violations from the Houthi side.
"We think that this is the only option for us to go down the path of peace and achieve peaceful resolution to the conflict in Yemen," he said.
Al Jazeera's Zeina Khodr, reporting from Biel, said the prisoner swap was "undoubtedly a positive development" in the civil war, which has killed almost 6,000 people and led to a humanitarian disaster in Yemen.
"The prisoner swap is taking place, mediated by tribes on the ground," she said.
She said the exchange did not appear to be in response to the Yemeni government's demands for Houthis to release a number of senior officials in their custody.
Earlier, Mokhtar al-Rabbash, a member of the prisoners' affairs committee, which is close to the Yemeni government, confirmed that an agreement was in place to swap 375 opposition fighters for 285 pro-Hadi soldiers.
An official of the Houthi-run prisons authority in the capital, Sanaa, said southern prisoners boarded buses on their way to the swap venue in central Yemen.
Witnesses in Aden said they saw buses guarded by local fighters travelling through the city, apparently heading to the exchange venue, the Reuters news agency reported.
Despite the swap deal, the two sides have accused each other of violating the truce, which includes a pause in air strikes by the Arab coalition.
The Saba news agency quoted Brigadier-General Sharaf Luqman, a spokesman for Yemeni forces loyal to Saleh, as saying that a "serious escalation by land, sea and air is taking place by the alliance in various areas".
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Luqman said strikes from the sea were being launched on the Red Sea port city of Hodeida, ground forces were carrying out attacks on Taiz, and air strikes by the Arab coalition had not stopped.
"We will not stay hand-tied but we will respond strongly towards the breaches that are taking place by the alliance and their mercenaries," Luqman said.
On the other hand, the Hadi-run sabanew.net news agency blamed the deaths of five loyalist fighters and three civilians on Houthi shelling in Taiz shortly after the ceasefire began.
The Saudi daily Al-Riyadh quoted Brigadier-General Ahmed al-Asseri, the Arab coalition spokesman, as saying that forces were committed to the ceasefire but were also ready to respond to any violation by the Houthis.
Despite the mutual accusations, Al Jazeera's Khodr said the two groups were still moving ahead with the peace talks in Switzerland.
"The fact that we are in day two and no one has walked out is progress in itself," she said.
Source: Al Jazeera and agencies