Houthi-backed forces attacked positions controlled by fighters loyal to Yemen President Abdurabu Hadi in Lahj, two senior Defense Ministry officials told CNN. The officials said that Houthis also launched rocket attacks on the airport in Aden, but no casualties or damages were reported.
Houthi officials said the land attacks come in retaliation for Saudi airstrikes in the first hour of the ceasefire. Those airstrikes occurred in the provinces of Hajjah and Saada, killing one civilian and injuring seven others, according to Houthi officials.
Mohammed Ali al-Houthi, the Houthi-assigned acting Yemeni President, told CNN on Monday that his group's forces did not accept or reject the Saudi-called ceasefire, since it was not sponsored by or coordinated with the United Nations. "We seek a real ceasefire with U.N. observation over breaches. Saudi (Arabia) is not serious about the ceasefire and that's why its airstrikes are fiercer today during the ceasefire than they were before the ceasefire was announced," al-Houthi told CNN.
By midday Monday, Saudi warplanes attacked Kahlan Military Base in Lahj province targeting Houthi military advances in Lahj. Three Saudi airstrikes also targeted positions controlled by the Houthis in Taiz.
According to the Houthi-controlled Defense Ministry, Saudi missiles killed at least nine allied fighters in Karsh district of Lahj. The fighters were being held hostage by the Houthis and the ministry said the airstrikes hit a location where allied hostages were being held.
The ministry said Yemeni forces continued their rocket attacks targeting two bases in Jazan province Monday afternoon.
Aden and Lahj provinces witnessed the most escalation with five Saudi airstrikes and Houthi clashes in the Aden districts of Daar Saad and Shiekh Othman.
Forces also clashed in northern Mareb province near AlMakhdarah district killing two tribal fighters loyal to President Hadi.
The Saudi-led coalition -- allied with the United States -- and Houthi rebels had agreed to a five-day ceasefire for humanitarian purposes.
The temporary truce was intended to allow for the delivery of medical and other humanitarian aid, deposed President Hadi said through Saudi Arabia's state news agency SPA. Hadi fled the country after Houthi rebels launched their offensive in Yemen.
Previous humanitarian ceasefires have struggled. One in May was only partially observed by the warring sides
, but it was enough to allow aid groups to get supplies to some of those in dire need. But another truce earlier this month failed to take hold, according to the United Nations
At least 3,000 people have been killed in Yemen since the conflict began in March, according to the United Nations. An estimated 21 million are in need of immediate humanitarian aid, and as many as 1 million people have fled their homes.
Houthi officials have said their forces "will only abide by the Saudi-announced ceasefire if Saudi Arabia does," said Mohammed Ali al-Houthi, the Houthi-declared acting president of Yemen.
"Unfortunately, previous ceasefires were not taken seriously and were not implemented on the ground."
The Houthi rebels, a Shiite minority, are backed by Iran. The Sunni-dominated Saudis have led a coalition in strikes against Houthi rebels and other groups.
Just hours after the ceasefire began, both sides accused each other of breaching the peace deal.
According to the Saudi-backed Yemeni government, Houthi rebels started artillery shelling almost immediately after the truce went into effect at midnight Sunday, according to a source close to the Yemeni government.
The source said Houthi militias shelled several areas in the central city of Taiz, including "many residential areas."
"These were initial and immediate indications of the failure of the ceasefire," the source said, but added that the Yemeni government considers the humanitarian truce ongoing despite the breach.
But the Houthi-controlled Defense Ministry accused a Saudi-backed coalition of violating the ceasefire, saying two airstrikes struck Hajjah and Saada provinces.
One person was killed and seven injured in the Hajjah strike on a medical center, which is used as a shelter by Houthi rebels, according to a senior official in the Houthi-controlled defense ministry.
No casualties were reported from the airstrike in Saada.
The source close to the Yemeni government would not comment on the airstrikes but reiterated that coalition has maintained that it had the right to respond to any military action by the Houthis