Stocks closed lower Wednesday as markets grappled with geopolitical uncertainty concerning the next move by the U.S. regarding Russia, Syria and North Korea. 

The S&P 500 was down 0.41%, the Dow Jones Industrial Average fell 0.33%, and the Nasdaq slipped 0.51%. The industrials and materials sectors were leading stocks lower by over 1%, with Caterpillar (CAT) - Get Report down 2.3% and Boeing (BA) - Get Report falling 1.4%.

Markets awaited an outcome from U.S. Secretary of State Rex Tillerson's visit to Moscow, where he addressed Russia's continued assistance of Syrian President Bashar al-Assad. Tillerson met with Russian President Vladimir Putin at the Kremlin for about two hours. The White House has charged Moscow with trying to cover up Syria's deadly chemical attack on April 4 and protect Assad.

Tillerson emphasized the Trump administration's stance that "the reign of the Assad family is coming to an end," in a press conference with Russian Foreign Minister Sergey Lavrov. Lavrov and Tillerson agreed to investigate the chemical weapons attack in Syria. However, there still appeared to be tension between the U.S. and Russia. 

"We have seen very alarming actions recently with an unlawful attack against Syria," Lavrov said of the U.S. airstrike in Syria, at the press conference. "We consider it of utmost importance to prevent the risks of replay of similar action in the future."

The U.S. last week launched 60 cruise missiles at a Syrian airbase, a move President Trump approved after Assad's regime killed dozen of civilians in the chemical attack.

Also on investor's minds is the strong language used by Trump regarding North Korea. In a tweet, Trump said, "North Korea is looking for trouble. If China decides to help, that would be great. If not, we will solve the problem without them! U.S.A." North Korea said Tuesday there could be "catastrophic consequences" after the U.S. ordered the USS Carl Vinson aircraft carrier and its battle group to waters off the Korean Peninsula.

"This has been a period in which the equity markets have flattened a little bit after the euphoria of the post election period and the anticipation of many of the promised elements of the new administration, namely fiscal measures and tax cuts infrastructure spending and rollbacks of regulation," Bill Northey, U.S. Bank's private client group chief investment officer, said in a phone interview. "The S&P 500 is right at its 50-day moving average so there has been a flattening in anticipation or a pause waiting for some of the elements to come to fruition."

Gold closed higher up 0.3% to $1,278.10 an ounce, the second day in a row, as investors moved into safe-haven investments.

Crude oil settled lower 0.5% to $53.11 a barrel Wednesday after the Energy Information Administration's weekly status report showed that U.S. crude oil inventories last week declined by 2.2 million barrels, the first weekly decline in a month. Gasoline fell 3 million barrels and distillates also dropped by 2.2 million barrels.

OPEC said in its monthly oil report on Wednesday that production decreased by 153,000 barrels a day, an average of 31.93 million barrels a day. OPEC agreed last year to reduce oil production by 1.2 million barrels a day in an attempt to increase oil prices. OPEC increased its forecast for the U.S. oil supply by 200,000 barrels a day to 540,000 barrels a day.

United Airlines (UAL) - Get Report  continues to deal with the fallout from the video of a passenger being violently dragged off a United plane for not voluntarily giving up his seat. 

CEO Oscar Munoz issued a second apology Tuesday, saying, "I continue to be disturbed by what happened on this flight and I deeply apologize to the customer forcibly removed and to all the customers aboard. No one should ever be mistreated this way." The apology on Tuesday came after Munoz, in a letter Monday, defended his employees saying the 69-year-old doctor was being "disruptive and belligerent."

United shares fell 0.2% on Wednesday after rising in earlier trading. 

Delta Air Lines (DAL) - Get Report  reported first-quarter adjusted earnings of 77 cents a share, topping forecasts by 2 cents. Delta said traffic in the first quarter rose 0.5% and capacity dipped 0.5%. The stock gained 1.4%.

Whole Foods (WFM) shares jumped 1.8% in trading after Bloomberg reported that Amazon (AMZN) - Get Report  considered taking over the grocer last year, but decided against making a move. The report follows news that activist investor Jana Partners built a 9% stake in Whole Foods ans is pushing for board changes.

Akzo Nobel (AKZOY) accused one of its biggest shareholders of planning to share price-sensitive information with rival PPG Industries (PPG) - Get Report and rejected calls for the dismissal of its chairman amid an increasingly hostile takeover showdown.

Fastenal (FAST) - Get Report shares dropped 7.1% Wednesday as the industrial and construction supplies company posted first-quarter earnings of 46 cents a share, meeting estimates. 

Neurocrine Biosciences (NBIX) - Get Report shares rose 23.6% after the Food and Drug Administration approved its drug Ingrezza to treat tardive dyskinesia, the only treatment for this disease.

Shares of BlackBerry (BBRY) popped 16% after the company announced it was awarded more than $814.9 million of royalty payments in an arbitration against Qualcomm (QCOM) - Get Report . Qualcomm shares fell 2.9%. 

Earnings will kick into high gear on Thursday as Citigroup (C) - Get Report , JPMorgan (JPM) - Get Report and Wells Fargo (WFC) - Get Report are scheduled to issue their quarterly reports.

Tractor Supply (TSCO) - Get Report  is down over 7% after the company announcing disappointing first-quarter results after the bell on Tuesday, saying it expected same-store sales to fall 2.2% and to miss profit expectations by as much as 8 cents.   

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As for U.S. economic data, import prices in the U.S. fell 0.2% in March, matching economists' estimates. Import prices rose 0.2% in February. Export prices rose 0.2% in March vs. expectations of an increase of 0.1%.