KUWAIT CITY — The United States pledged today to send an additional $380 million in aid this year to help victims of Syria’s brutal civil war, but warned international efforts to ease their suffering will fail if Syrian President Bashar Assad refuses to let humanitarian assistance get to people who need it.

The U.S. is the largest single international donor to the Syrian crisis. But its new funding commitment, announced by U.S. Secretary of State John Kerry, is only a fraction of the $6.5 billion the United Nation hopes to raise for aid to Syria in 2014.