The Geneva-based SAM Organisation for Right and Liberties, in its report published on Wednesday, said the killings were part of 1,937 violations committed throughout the country in December, including physical assaults, violation to press freedom, torture and arbitrary detention.
The violations were perpetrated by "Houthis militia, Arab Coalition air force, military formations and groups loyal to the legitimate government", it said.
"SAM Organization for Rights and Liberties condemn all crimes included in this report which are considered as gross violations of the international humanitarian law and human rights law," it added.
Some 279 civilians died as a result of Saudi Arabia-led coalition air raids, in addition to the 121 killed by Iranian-backed Houthi fighters, the report noted.
Saudi Arabia has been at war in Yemen since March 2015, when a coalition led by the oil-rich kingdom launched a campaign of aerial bombardment, aimed at countering Iranian-backed Houthi rebels and reinstating the government of President Abd-Rabbu Mansour Hadi.
The rights monitor urged the Saudi-led coalition to "avoid targeting civilians and to review its rules of combat in accordance with the international laws and conventions".
A recent UN report on human rights abuses related to foreign intervention in Yemen documented a number of civilian casualties inflicted by Saudi-led coalition bombing.
The panel examined 10 air attacks in 2017 that killed 157 people and found that the targets included a migrant boat, a motel and five residential buildings, according to a copy of the report shown to Al Jazeera.
"Even if, in some cases, the Saudi-led coalition had targeted legitimate military objectives, the panel finds it highly unlikely that the IHL (International Humanitarian Law) principles of proportionality, and precautions in attack were met," the report stated.
To date, more than 10,000 people have died in the war in Yemen, according to the UN.
Yemen's cholera outbreak, a direct consequence of the conflict, has claimed about 2,000 lives and affected more than one million people since April 2017.
More than two million people have been displaced since the war began.