The government said the assault, which left 10 soldiers dead and several wounded, was aimed at cutting off a key route to the blockaded city of Taiz and undermined peace efforts.
Two separate attacks on Yemen's Zinjibar and Ataq leave nine soldiers dead while three other militants were also killed in the gunfire.
The announcement from UN Special Envoy for Yemen came only a few hours before the original truce was set to expire.
The authorities gave no indication of foul play in the blast that killed at least two people, wounded 120 others and damaged several buildings and stores.
Three people killed in main Sanaa city after rebels shoot down a spy drone of Saudi-led military coalition, Houthi officials say.
Rebels "will consider the request for an extension of the truce, which ends on June 2," says Saba news agency, which is affiliated with the Houthis.
The International Committee of the Red Cross has facilitated the release and repatriation of nearly 100 Yemeni prisoners.
Yemen's government has blamed Houthi rebels for the postponement of the flight meant to transport passengers in need of medical treatment to Jordan's capital.
Houthis sign what the UN describes as an "action plan" to end and prevent recruiting or using children in armed conflict.
During the two-month truce, the Saudi-led coalition will allow 18 vessels carrying fuel into the port of Hodeida, and two commercial flights a week from the Yemeni capital to Jordan.
Coalition says it will halt military operations to help create a positive atmosphere during the holy Muslim month of Ramadan, in a truce Houthi rebels rejected, calling it "meaningless".
Saudi-led coalition has said attacks targeted Aramco's water desalination plant and power station in the southern city of the kingdom, and a gas facility in Khamis Mushait.
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