Story highlights A five-day humanitarian ceasefire went into effect at midnight Sunday

Both a Saudi-led coalition and Houthi rebels in Yemen are accused of violating the ceasefire

At least 3,000 people have been killed this year in Yemen, the United Nations says

Sanaa, Yemen (CNN) Day 1 of a scheduled five-day humanitarian ceasefire in Yemen witnessed more than a dozen Saudi airstrikes and Houthi clashes on numerous fronts as the dueling sides both breached the long-awaited break.

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.

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