More: The Los Angeles Port head added on Thursday that U.S. exporters are also getting "hit hard" by retaliatory tariffs amid Trump's trade war. Seroka said the sectors include agriculture, heavy-duty manufacturing and information technology services.

"U.S. ag exporters are having an especially challenging time, so much so that in March, China bought more soybeans from Brazil in one month than ever in their history," Seroka said.

Meanwhile, major retailers have told Seroka that they have about a six- to eight-week supply of inventory but that "will quickly dry up." The Los Angeles Port is the major point of entry for cargo ships from China and Southeast Asia into the U.S.