The International Committee of the Red Cross (ICRC) said a hospital it supported in Saada had received dozens of casualties after the attack.
He criticized Riyadh for denying Iranian delegates entry visas for OIC summits, saying such statements "are compiled and released unilaterally and unjustly under pressure from Saudi Arabia".
The ballistic missile was sacked from the northern Yemeni province of Amran towards the southern Saudi city of Jizan, said a coalition statement published by the official Saudi Press Agency.
Nearly 10,000 people - two-thirds of them civilians - have been killed since the Houthi movement took control of much of the north of the country, including the capital Sanaa, in 2014, forcing President Abdrabbuh Mansour Hadi into exile in neighbouring Saudi Arabia.
Later, air strikes were reported in the rebel-held Yemeni capital, Sanaa.
The bus was struck as it was driving through a market in the rebel-held province of Saada, according to the Houthi-run Al-Masirah TV.
"Under global humanitarian law, civilians must be protected during conflict", the ICRC said.
Houthi spokesman Mohammed Abdul-Salam said the coalition showed "clear disregard for civilian life" as the attack had targeted a crowded public place in the city.
Col. Al-Maliki affirmed that the leaders and militants responsible for launching ballistic missiles and targeting civilians would be held accountable under the Coalition's efforts to prevent terrorist elements from compromising regional and worldwide security.
Saudi Arabia says this was a legitimate military operation, targeting Yemeni rebels backed by Iran, its arch enemy.
Thursday's attack marks a grim milestone in a war that lately has seen yet another escalation, as coalition-backed Yemeni government forces have made a recent push to reclaim the key port city of Hodeidah from rebels.
The United Nations has repeatedly criticized the actions of the coalition.
Saudi Arabia and Sunni Muslim allies intervened in Yemen's war in 2015 against the Houthis, who control the most populous areas of Yemen, including the capital Sanaa, and drove the internationally recognized government into exile in 2014.
The Saudi government issued a statement on August 9 and described the strike as "a legitimate military action" and was "conducted in conformity with the global humanitarian law".
The United States is helping the coalition, the only party in the conflict to use war planes, with refueling, intelligence, and billions in weapons sales. Just last week at least 55 people died in the city during a particularly bloody exchange of fire in residential areas and outside a hospital.
Yemen's war has left almost 10,000 people dead since 2015 and unleashed what the United Nations describes as the world's worst humanitarian crisis.
In another video, which appears to show the immediate aftermath of the strike, several lifeless bodies of children lay under a blown-up bus.