By Costas Paris 

Trans-Pacific trade tensions weighed on volumes at Southern California ports in May, prompting warnings that the yearlong tariff spat between the U.S. and China is rattling supply chains.

Combined inbound loaded containers at the ports of Los Angeles and Long Beach, the neighboring ports that comprise the largest U.S. gateway for seaborne trade, fell 6.3% last month from a year ago. The ports handled 48,256 fewer loaded import containers than they did in the same month last year.

Combined exports at the two ports also fell 7.4%.

The decline, in a month when maritime operators usually are gearing up for the year's peak seasonal shipping volumes, suggested that importers are pausing orders for goods after accelerating imports for several months to get ahead of rising tariffs launched by Washington and Beijing.

"Escalating tariffs pushed retailers to order early, warehouses are brimming with inventory and carriers are managing their vessels to deal with reduced demand," said Mario Cordero, executive director of the Port of Long Beach, where loaded import container volume fell 19.5% last month from same month a year ago.

"We are hopeful Washington and Beijing can resolve their differences before we see long-term changes to the supply chain that impact jobs in both nations," Mr. Cordero said in a statement.

Last month, the Trump administration raised tariffs on $200 billion of Chinese goods, including furniture and automotive parts, to 25%. China responded with tariffs on $60 billion in U.S. goods. Washington has also threatened to impose levies on all remaining Chinese exports worth around $300 billion.

The Global Port Tracker report by the National Retail Federation and Hackett Associates released this week estimated that container imports into major U.S. ports overall rose 3% in May. The retail group also scaled back its projection for imports in the coming months, saying U.S. ports would see a combined 900,000 fewer inbound containers in June, July and August than had been forecast in May.

"With a major tariff increase already announced and the possibility that tariffs could be imposed on nearly all goods and inputs from China, retailers are continuing to stock up while they can to protect their customers as much as possible against the price increases that will follow," NRF Vice President for Supply Chain and Customs Policy Jonathan Gold said in a statement.

Write to Costas Paris at costas.paris@wsj.com

(END) Dow Jones Newswires

June 12, 2019 10:56 ET (14:56 GMT)