Norwegian Refugee Council says sniper and explosive attacks have been growing despite an easing of air attacks amid a three-month-old truce in Hudaida, as the devastating conflict intensifies elsewhere.
An international charity says fighting in two Yemeni provinces has killed at least 350 civilians in the past three months.
The Norwegian Refugee Council (NRC) said on Monday that civilian deaths have doubled in Hajjah and Taiz provinces, even as casualties in the flashpoint city of Hudaida were down since a shaky ceasefire came into effect there in December.
Mohamed Abdi, NRC director in Yemen, says, "Fighting is intensifying in other parts of the country with a devastating impact on civilians," citing 164 people killed in Hajjah and 184 in Taiz.
"The reduction in violence seen in Hudaida through recent months has been counteracted by escalations in other parts of the country," said Abdi.
He called on the combatants to return to the negotiating table and extend their ceasefire across all of Yemen to "put an end to this untold suffering for Yemeni civilians."
The NRC's analysis found that while civilian casualties from air strikes were down by half since December, attacks from roadside bombs, mines and snipers were on the rise.
The group said that 13 schools were among the buildings attacked over the past three months and that 80 percent of Yemen's population – 24 million people – still required humanitarian assistance.
The World Health Organization estimates that nearly 10,000 people have been killed in Yemen since March 2015, when Saudi Arabia and the United Arab Emirates – with the logistical and political backing of the United States – unleashed air power against the Houthi rebels.
Rights groups say that the death toll could be far higher. Save the Children has estimated that 85,000 Yemenis under five years old may have died of starvation.