By Nick Timiraos 

The U.S. trade gap widened in September as exports fell to a five-month low, a sign of weaker demand for U.S.-made goods that underscores concerns about a global economic slowdown.

The trade deficit rose 7.6% to a seasonally adjusted $43 billion in September from the prior month's deficit of $40 billion, the Commerce Department said Tuesday. Exports decreased 1.5% from August while imports were almost unchanged.

The report showed softer demand for U.S. goods such as industrial supplies and autos. The figures prompted many economists to trim their forecasts for growth in the third and fourth quarters.

Nonpetroleum imports rose to their highest level on record, boosted by a nearly $2 billion jump in imports of cellphones that economists said likely reflected the introduction of Apple's iPhone 6. Lower energy prices sent petroleum imports to their lowest level in nearly five years.

The Commerce Department reported last week that gross domestic product, the broadest measure of goods and services produced across the economy, expanded at a 3.5% annual rate during the third quarter.

That estimate was based on projected figures for Tuesday's trade report. Commerce will revise its third-quarter GDP estimate, incorporating the new September trade numbers, later this month. The initial GDP estimate showed that exports boosted GDP growth by more than one percentage point while a drop in imports added 0.3 percentage point to growth.

Tuesday's trade report prompted economists at Morgan Stanley, Macroeconomic Advisers and Royal Bank of Scotland to cut their third-quarter growth estimates to 3%. Economists also could begin to reduce their fourth-quarter forecasts to around 2% from 3%, said analysts at Mizuho Securities.

The report highlights concerns about how the U.S. economy will fare if the dollar continues to strengthen against foreign currencies and if growth slumps in Europe and Asia. Many economists expect trade to drag on growth through the middle of next year because a stronger dollar makes U.S. goods more expensive overseas, denting exports.

Exports to Europe swung from a 4.6% increase in August to a 7% drop in September, the lowest level since February 2013. Exports to China swung from a 3% increase in August to a 2% drop in September, the lowest since last December.

"Exports looked weak across the board," said Michelle Girard, chief U.S. economist at RBS. While analysts might attribute September's drop to a stronger dollar, "the lags between currency appreciation and weaker exports tend to be longer," she said in a note to clients Tuesday.

Meanwhile, imports from China rose 13% to an all-time high, and the trade deficit with China hit $35.6 billion in September, also a record.

Exports have increased this year because of a domestic energy-production boom from North Dakota to Texas aided by advanced drilling techniques such as hydraulic fracturing. But falling oil prices have raised questions about the pace of investment in the domestic energy sector.

Oil prices fell Monday to their lowest point in more than two years after Saudi Arabia cut prices for crude sold to the U.S., which could pave the way for future price declines and put pressure on U.S. energy producers.

Petroleum imports fell 3.2% in September to $26.4 billion, the lowest level since November 2009. Exports of petroleum fell 12.5% to $12.4 billion, the lowest level since April.

Write to Nick Timiraos at nick.timiraos@wsj.com

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