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Stock losses accelerated by mid-morning Thursday as a reading on manufacturing activity in the U.S. worsened. 

The S&P 500 was down 0.56%, the Dow Jones Industrial Average fell 0.47%, and the Nasdaq rose 0.4%.

The August ISM Manufacturing Index fell into contractionary territory at 49.4 from 52.6 in July, well below an expected reading of 52.2. A strong U.S. dollar and weak global demand have impeded manufacturing growth for months. 

A separate reading on manufacturing showed a slight drop in August. The August U.S. PMI Manufacturing Index eased to 52 from 52.9, though output growth remained at its July high, its best level in eight months. 

Costco (COST) - Get Report fell 3% after reporting a flat reading on comparable-store sales for August and the recent quarter. Excluding gas and currency exchange, sales rose 2% for August and 3% over the quarter. Analysts had expected same-store sales to increase 1.3% in August and 3.4% adjusted to exclude gas.

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Ford (F) - Get Report shares fell 1% after the automaker reported an 8.4% decline in sales over August. Passenger car sales slumped 25% to 52,148 last month, while truck sales declined 1.6%. Fiat Chrysler (FCAU) - Get Report reported a far better August with total unit sales up 3% and passenger car sales down 13%. Sales at General Motors (GM) - Get Report in August fell 5.2%. 

Campbell Soup (CPB) - Get Reportslipped 5% following a disappointing fourth quarter. The packaged food company swung to a loss of 26 cents a share from earnings of 5 cents a share a year earlier. Adjusted earnings of 46 cents a share missed consensus by 50 cents. Campbell's revenue of $1.687 billion came in short of consensus of $1.694 billion.

Joy Global (JOY) moved slightly higher after reporting a quarter in which weak demand for mining equipment hurt results. Total bookings fell 17%, dragged on by a 46% slump in original equipment orders. Japanese company Komatsu announced in July that it would purchase Joy Global for $2.9 billion.

Box (BOX) - Get Report was 1% higher after reporting a narrower quarterly loss than expected and a double-digit jump in revenue. The file-sharing developer reported a loss of 14 cents a share over its second quarter, a nickel narrower than forecast. Revenue jumped just over 30% to $95.7 million. (CRM) - Get Report fell 5% after issuing light guidance for its current quarter. The cloud software developer expects current-quarter earnings of 20 cents to 21 cents a share on revenue no higher than $2.12 billion. Consensus was for earnings of 24 cents a share and sales of $2.124 billion.

Infoblox (BLOX) was 1% higher after narrowly beating profit and sales estimates over its recent quarter. The cybersecurity firm earned 8 cents a share, 2 cents above estimates, while revenue of $86.3 million bested estimates by $1.8 million. Infoblox jumped over Wednesday's session on reports it has begun the process of selling itself after pressure from activist investor Starboard Value.

The number of new claims for unemployment benefits in the U.S. increased by 2,000 to 263,000 in the past week. Analysts had expected an increase to 265,000.

Productivity over the second quarter fell 0.6%, according to the Bureau of Labor Statistics, which came in as analysts had expected. Output increased 1.1%, while hours worked rose 1.7%.

The official jobs report for August, the most closely watched release of the month, will be out on Friday. Economists expect the pace of jobs growth in August to slow after blockbuster readings in June and July. Analysts at TD Securities expect 189,000 jobs to have been added to the U.S. economy, while the unemployment rate will dip to 4.8%. The U.S. economy added 255,000 jobs in July.

Stocks ended August on a dour note Wednesday as another selloff in crude oil pressured the energy sector and speculation over interest rates continued to cause jitters. The S&P 500 ended August with a slight monthly loss.