UNITED NATIONS, Feb 12 (Reuters) - The United States on Sunday called for the United Nations Security Council to "vote immediately" to authorize the delivery of U.N. aid to rebel-held northwest Syria through more border crossings from Turkey after last week's deadly earthquake.

Since 2014 the U.N. has been able to deliver aid to millions of people in need in the northwest part of war-torn Syria through Turkey under a Security Council mandate. But it is currently restricted to using just one border crossing.

"Right now, every hour matters," Linda Thomas-Greenfield, the U.S. ambassador to the U.N., said in a statement to Reuters. "People in the affected areas are counting on us."

"We cannot let them down - we must vote immediately on a resolution to heed the U.N.'s call for authorization of additional border crossings for the delivery of humanitarian assistance," she said. "It's time to move with urgency and purpose."

U.N. aid chief Martin Griffiths, who is in Turkey and is due to visit Syria, told Sky News on Saturday that he would ask the Security Council to authorize aid access through two more border crossings, arguing there is "a very clear humanitarian case."

U.N. Secretary-General Antonio Guterres on Thursday pushed for more access.