WASHINGTON, Aug 24 (Reuters) - The United States has evacuated more than 4,000 American passport holders plus their families from Afghanistan in the ongoing airlift from Kabul, a senior State Department official said on Tuesday. There was a delay in reporting the figures because U.S. officials were focused on getting people out of Kabul as quickly as possible, the official said. "We expect that number to continue to grow in the coming days," said the official, who spoke on condition of anonymity.