By Paul Hannon in London and Costas Paris in New York 

World trade flows have sagged again in 2019, a sign that higher U.S. tariffs and other trade barriers are cooling growth as leaders from the Group of 20 large economies prepare to meet in Japan.

Most of the measures have been introduced or proposed by the U.S. or by countries retaliating against Washington's tougher approach to tackling its trade deficits. More could come. The U.S. government for instance has detailed nearly $300 billion in new Chinese imports that would face 25% levies as early as this summer.

If those levies go ahead, economists at UBS estimate tariff levels could return to where they were in 2003, with the shipping industry carrying cars, clothing, electronics and other manufactured goods around the world among the businesses bracing for a substantial impact.

Denmark's A.P. Moller-Maersk, which owns roughly 20% of all container capacity, said the spat between the U.S. and China may cut growth in global container volumes by a third this year. Shipping industry measures show freight prices have already been slipping this spring because of declining volumes.

"The tariffs are the biggest negative risk that can seriously hit volumes and earnings," said Jonathan Roach, a container shipping analyst at London-based Braemar ACM. "The optimism we had just a few weeks ago when we thought a trade deal would be signed between the U.S. and China has evaporated. There is a lot of uncertainty out there."

U.S. President Trump and Chinese President Xi Jinping are set to meet on the sidelines of the G-20 gathering. While that meeting isn't expected to result in a trade agreement, it may help restart talks that broke down in May.

Meanwhile, concern about the impact on global trade flows is deepening.

Figures released Tuesday by the CPB Netherlands Bureau for Economic Policy Analysis indicated that the total volume of goods moving across borders fell 0.7% in April from March, having dropped by 0.2% in the first three months of the year. The Dutch economy has been highly dependent on trade for centuries, and the government's economic research body pays special attention to changes in trade flows.

The April drop in trade flows was driven by a 2.6% decline in imports to the U.S., and a 5.3% slump in exports from Asia's developing economies, which includes China. Economists say those declines partly reflect new barriers to trade, chiefly the higher tariffs that have been imposed since early 2018.

In a report prepared for G-20 leaders, who will meet in Osaka on Friday and Saturday, the World Trade Organization said the number of measures restricting trade that were imposed between October 2018 and May 2019 was 3 1/2 times the average since it started to monitor policy in 2012. The measures affected $335.9 billion worth of potential imports, the second-largest total on record after the six months through September 2018.

"This will have consequences in increased uncertainty, lower investment and weaker trade growth," said Roberto Azevêdo, the WTO's director-general. "We urgently need to see leadership from the G-20 to ease trade tensions."

The trade slowdown appears to have weakened factory output around the world. The Dutch research institute said that global industrial production was 0.8% lower in April than in March, having increased by just 0.1% in the first quarter.

June measures of activity in the U.S., eurozone and Japanese manufacturing sectors also pointed to a slowdown. By contrast, activity in the services sector, which isn't directly affected by higher tariffs, continued to grow at a robust pace. Taken together, Capital Economics calculates that the purchasing managers' indexes for the three months through June point to the weakest growth across developed economies since 2012.

Chinese shipping executives say they have withdrawn box capacity in the trans-Pacific route since the first round of tariffs were introduced last summer. Cosco Shipping, the world's third-biggest box-ship operator, has removed 10% of its carrying capacity on the route since the end of last summer after the first U.S. tariffs were introduced.

Container imports into the five biggest U.S. West Coast seaports, the main gateways for U.S.-Asia trade, fell a combined 5.3% in May, according to Beacon Economics, a steep pullback from the surge in inbound business in earlier months as importers sought to rush goods into the country ahead of new tariffs.

Export loads from the ports, which include the agricultural goods targeted by China in retaliatory actions, fell 5.1% from a year earlier.

The upheaval in global trade is weighing on airlines. The International Air Transport Association this month trimmed by 21% its combined profit forecast for airlines to $28 billion from $35.5 billion six months ago, largely reflecting a slump in airfreight demand and cost pressures.

In December, IATA expected cargo shipped by air to grow 3.7% this year. It cut that prediction to zero growth this month. It also cut pricing expectations for those items shipped to no growth from a 2% yield increase expected in December.

"The last six months have been a bit of a disaster for air cargo. We've seen a really sharp fall in volumes," IATA chief economist Brian Pearce said this month. The imposition of tariffs by the U.S. and China on imports have led to sharp drops in May airfreight traffic, measured by tons over distance flown, down 4.7% compared with the year prior figure.

Deutsche Lufthansa AG last month said it would offer fewer cargo flights to adjust to lower demand.

--Robert Wall in London and Paul Page in Washington contributed to this article.

Write to Paul Hannon at paul.hannon@wsj.com and Costas Paris at costas.paris@wsj.com

(END) Dow Jones Newswires

June 25, 2019 12:45 ET (16:45 GMT)