Amnesty International accused the Saudi-led coalition of "turning a blind eye to civilian deaths" as it reported four children were among six burned to death in strikes on Ibb, in central Yemen.
But the spokesman for the Saudi-led coalition said it does not intend to kill civilians even though the Huthis had moved fighters into villages.
"Collateral damage can happen... but I confirm to you that the coalition takes all care," said Brigadier General Ahmed Assiri.
The UN children's fund said that at least 62 children had been killed and 30 injured during the fighting in Yemen over the past week.
"Children are in desperate need of protection, and all parties to the conflict should do all in their power to keep children safe," said UNICEF's Julien Harneis.
- 'Day of terror' -
Loud blasts were heard overnight in Sanaa when coalition forces hit a missile depot belonging to the renegade Republican Guard, which is loyal to ousted president Ali Abdullah Saleh.
"The bombing was the heaviest I have ever heard in my life. The explosions lit up the skies of Sanaa," said 30-year-old resident Amr al-Amrani.
On Tuesday, air strikes targeted two Huthi-held camps and Guard soldiers in the southern town of Daleh, a Guard air base in the southwestern city of Taez and the Huthi stronghold of Dhammar, south of Sanaa.
On the ground, deadly clashes have broken out between the rebels and tribes, militiamen and residents who oppose their power grab.
Doctors Without Borders (MSF) said it had received more than 550 patients in the southern city of Aden since March 19 as a result of fighting.
"We urgently need to find ways to get humanitarian relief and personnel inside the country," said MSF's Greg Elder.
The UN said that since Friday, at least 93 civilians had been killed and 364 injured.
"The situation in Yemen is extremely alarming, with dozens of civilians killed over the past four days," UN High Commissioner for Human Rights Zeid Ra'ad Al Hussein said.
"The country seems to be on the verge of total collapse."
He denounced reported attacks by Huthi-linked fighters on three hospitals in Daleh that caused an unknown number of casualties.
A spokeswoman for European Union foreign policy chief Federica Mogherini said the conflict was having "terrible consequences" for civilians.
"We call on all parties to the conflict to ensure the protection of civilians, that civilian infrastructure is not directly targeted," the EU said.
The International Committee of the Red Cross said it was trying to negotiate the safe arrival of a plane stocked with medical supplies to treat up to 1,000 people.
"There are casualties across the country," said the ICRC's Cedric Schweizer.
- War of words -
Trading blame for the escalation, Saudi Foreign Minister Prince Saud al-Faisal charged that the Huthi rebels and their ally, Saleh, had "decided with the support of Iran to destabilise Yemen".
"We are not warmongers, but when they beat the drums of war we are ready," he said.
Tehran hit back, accusing Riyadh of jeopardising the entire Middle East.
"The fire of war in the region from any side... will drag the whole region to play with fire," said Deputy Foreign Minister Hossein Amir-Abdollahian.
Iranian state media rejected as "utter lies" claims Tehran had sent arms to Yemen, saying it had only dispatched non-military aid.
The war of words came as marathon nuclear talks between Tehran and world powers reached a crucial phase in Switzerland.
The coalition has vowed to keep up the strikes until the Huthis end their uprising against President Abedrabbo Mansour Hadi, who has fled to Saudi Arabia.
But with little sign of an imminent halt to the conflict, the UN said it relocated its peace envoy for Yemen, Jamal Benomar, to Jordan at the weekend and pulled its last 13 foreign employees from the country.
The European Aviation Safety Agency, meanwhile, issued an advisory for airline companies to avoid Yemeni airspace.