Stocks remained near lows by mid-afternoon Thursday as worries over crude oil continued to pressure markets.

The S&P 500 slid 0.56%, the Dow Jones Industrial Average fell 0.68%, and the Nasdaq was down 1%.

Crude oil declined on reports a meeting between major oil producers set for March 20 was unlikely to happen. Members of the Organization of Petroleum Exporting Countries and non-member Russia are reportedly waiting on Iran to confirm whether it would be open to a production-freeze agreement. West Texas Intermediate crude was down 1.9% to $37.57 a barrel.

"The global petroleum markets are testing the downside in Thursday trade on OPEC comments suggesting a proposed March 20 producer summit in Moscow may not happen," said Citi's Timothy Evans. "With Saudi Arabia firm on maintaining production and Iran determined to increase output to prior levels, there wasn't much hope of a breakthrough in any case."

Good vibes over the European Central Bank's stimulus measures also faded in the afternoon session as investors digested the likelihood of no further rate hikes. 

"From today's perspective and taking into account the support of our measures to growth and inflation, we don't anticipate that it will be necessary to reduce rates further," ECB President Mario Draghi said in a press conference. 

The ECB introduced more stimulus to revitalize the eurozone economy on Thursday morning. The ECB lowered the benchmark interest rate to zero, while cutting its deposit facility rate to minus 0.4%. The central bank also expanded its monthly asset purchases to 80 billion euros, beginning next month.

"This comprehensive package will exploit the synergies between the different instruments and has been calibrated to further ease financing conditions, stimulate new credit provision and thereby reinforce the momentum of the euro area's economic recovery and accelerate the return of inflation to levels below, but close to 2%," Draghi said.

The majority of economists surveyed expected further monetary easing measures from the ECB, especially another cut to the deposit rate and a boost to its bond-repurchasing program.

"The ECB is willing to take bold additional steps to encourage sustainable growth in the eurozone, thereby increasing investor confidence and making those buying opportunities look even more appealing," Nigel Green, deVere Group's CEO, wrote in a note.

The U.S. labor market continued to show its strength as weekly jobless claims fell. The number of new claims for unemployment benefits fell 18,000 to 259,000 in the past week, the lowest level in five months. The less-volatile four-week average fell 2,500 to 267,500, its fifth week in decline.

Chinese stocks slid after consumer inflation rose at a faster-than-expected pace, a troubling sign as the country's central bank attempts to stabilize falling economic growth. Inflation rose 2.3% in February, its fastest pace since mid-2014 and above estimates of 1.9% growth. Producer prices fell 4.9%, its 48th month in decline. The Shanghai Composite fell 2%.

Dollar General (DG) added more than 4% after beating quarterly estimates and raising its quarterly dividend. The discount retailer reported a 2.2% increase in same-store sales, driven by revenue in candy and snacks and other perishables. The company increased its dividend 14% to 25 cents a share and also said it expects to repurchase $1 billion in shares in fiscal 2016.

Nasdaq (NDAQ) has agreed to buy U.S. options exchange International Securities Exchange for $1.1 billion from Deutsche Boerse. Deutche Boerse's sale could be a move to bolster its balance sheet as it seeks a merger with the London Stock Exchange. Deutsche has written down the International Securities Exchange several times since acquiring it in a $2.8 billion deal in 2007.

Square (SQ) climbed more than 1% after reporting a 49% surge in quarterly sales in its first earnings report since going public. The payments site reported a loss of 34 cents a share, more than double estimates, though sales of $374.4 million came in above forecasts.

Gilead (GILD) and Merck (MRK) shares were on watch as the two drugmakers prepare to battle one another in a California courtroom this week. Merck is arguing that Gilead's hepatitis C drugs Sovaldi and Harvoni infringe upon two of its patents and so deserves royalties. The drugs generated sales of $12.5 billion in the U.S. last year.

Box (BOX) jumped more than 11% following positive guidance for the full year. The cloud-computing company anticipates a loss of 83 cents to 85 cents a share in fiscal 2016, narrower than an estimated 88-cent loss. Box reported a fourth-quarter loss of 26 cents a share, better than a forecast loss of 29 cents.

Nuance Communications (NUAN) shares were slightly lower despite agreeing to repurchase $500 million of its stock from billionaire investor Carl Icahn's Icahn Group. The company agreed to pay $19 a share, an approximate 6% discount to its Wednesday closing price.

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