United Nations: Preliminary reports said 13 people were killed and at least 70 others wounded in clashes in Yemen's Aden and Abyan governorates in the past two days, a UN spokesman said.

Stephane Dujarric, spokesman for UN Secretary-General Antonio Guterres, said on Thursday that streets in Aden are empty and flights to and from the airport are temporarily suspended, Xinhua news agency reported.

UN Humanitarian Coordinator in Yemen Lise Grande on Thursday called on everyone who is fighting to respect their obligations under international humanitarian law.

She also repeated her appeal for support to the humanitarian response in Yemen.

Dujarric said this year's humanitarian response plan requires 4.2 billion U.S. dollars, but only 34 percent is funded.

The program assists more than 20 million Yemenis, including 10 million people who have relied entirely on humanitarian assistance to meet their basic needs, he said.

Yemen faces the world's worst humanitarian crisis. Almost four out of five people require some form of humanitarian assistance or protection, the spokesman said.

Dujarric said if donation pledges are not met soon, 22 major programs could be closed in the coming weeks.