HOMS, Syria—Hundreds of evacuees from this besieged city were detained by the Syrian regime as soon as they were rescued, said the top United Nations official here, who said the U.N. now has no control over their fate.

The disclosure came as President Barack Obama and his visiting French counterpart, François Hollande, expressed dismay in Washington about the humanitarian crisis in Homs and elsewhere in Syria. Mr. Obama described the situation as "horrendous" and asserted that the country was "crumbling"...