The U.N. says authorities in Yemen have detained over 2,000 migrants in the southern city of Aden since Sunday, including at least 400 children.

U.N. spokesman Stephane Dujarric said Friday the International Organization for Migration is "deeply concerned" about the conditions in which the migrants, mainly from Ethiopia, are being held.

The U.N. migration agency is engaging with Yemeni authorities to ensure that the migrants' critical needs such as basic health care, food, water and sanitation are met, he said. It is also urging authorities "to find safer alternatives to detention."

Yemeni security officials said Wednesday that police had detained at least 5,000 migrants in the previous 10 days who were attempting to cross to Saudi Arabia. The officials said most were from African countries.