Saudi-led airstrikes killed civilians Sunday in Yemen's second city of Aden, as the U.N. envoy arrived in the capital for talks on a cease-fire plan with Houthi rebels.
The number of dead in the latest attack is unclear, media sources report between eight and 30 civilians killed.
The death toll of violence between pro-government forces and the Houthis, combined with Riyadh's three month air campaign against the militia, continues to increase, despite international efforts to negotiate a truce.
"We are intensifying our efforts to reach a peaceful resolution to this crisis that, in truth, as you know, has resulted in a humanitarian catastrophe for the Yemeni people," U.N. envoy Ismail Ould Cheikh Ahmed said upon landing in Sana'a.
"We were optimistic, but the central issue that we are focusing on is a long-term peaceful resolution and to bring back the issues to the negotiating table so the Yemeni people can return to a peaceful political process," he said.
The Houthis overtook parts of the country in late 2014, sparking months of clashes and the exile of internationally-recognized president Abdu Rabu Mansour Hadi.
Ould Cheikh Ahmed's visit to Yemen follows talks in Riyadh last week with the exiled government.