Stock futures were slightly lower on Friday, Sept. 15, as geopolitical worries spiked following another missile launch from North Korea and a terrorist attack in London. 

Dow Jones Industrial Average futures were down 0.02%, S&P 500 futures fell 0.08%, and Nasdaq futures declined 0.07%. 

An explosion on a London Underground train Friday morning was declared a terrorist incident by British police. At least 18 people have been injured when a possible explosion took place on the District Line at the Parsons Green stop in West London. Reports indicated the explosion was caused by a homemade bomb, placed into a bucket. 

In a statement following the incident, London Mayor Sadiq Kahn said the city would "never be intimidated or defeated by terrorism." He added, "I urge all Londoners to remain calm and vigilant."

North Korea again ignored international calls to cease its ballistic tests after launching a missile that flew over Japan for the second time in a month. The overnight launch flew more than 3,700 kilometres, Japan's Chief Cabinet Secretary Yoshihide Suga said, and landed in the sea around 2,000 kilometres east of the island of Hokaido.

U.S. Secretary of State Rex Tillerson said that "continued provocations only deepen North Korea's diplomatic and economic isolation", and urged tougher sanctions. The U.N. Security Council is slated to meet later today to discuss the issue. The Council on Monday had voted for new sanctions against North Korea. 

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Crude oil prices stabilized on Friday after settling at their highest level in more than six weeks a day earlier. This week's rally kicked off on signs of reduced global production. Global oil supplies have hit a snag in recent weeks as Hurricane Harvey limited output in the U.S. and declining activity in the Middle East helped the Organization of Petroleum Exporting Countries keep to a previous deal to limit production.

The weekly Baker-Hughes rig count will be released as normal at 1 p.m. ET. Baker Hughes reporteda total 944 active rigs in the U.S. in the week ended Sept. 8. 

West Texas Intermediate crude was flat at $49.90 a barrel on Friday.

U.S. retail sales fell in August as weaker auto sales weighed on the headline number. Retail sales fell by 0.2%, though rose by 0.2% when excluding autos. Excluding autos and gas, retail sales fell by 0.4%.

Business conditions in the New York region dipped in September, though at a narrower pace than expected. The Empire State Survey fell by 0.8 points to 24.4. Economists expected a sharper decline to 19. 

It's a busy day on the economic calendar. A preliminary reading for consumer sentiment for September, and business inventories for July also will be released Friday.

Shares of Oracle Corp. (ORCL) fell 3.9% in premarket trading on Friday after the computer software giant signaled slower growth in its cloud business. The stock initially rose after Oracle reported better-than-expected earnings and revenue in its fiscal first quarter on Thursday but then declined after the company said it expected total cloud revenue to increase 39% to 43% in the fiscal second quarter, down from last quarter's growth of 51.4%.

The airline sector was under pressure on Friday after JPMorgan downgraded American Airlines Group Inc. (AAL) and United Continental Holdings Inc. (UAL) to neutral on weaker pricing and higher fuel expenses. Analysts are uncertain the two airlines can meet fourth-quarter estimates. American fell 2.4% and United declined 2.6%.

Trading on Friday could be volatile during the quadruple-witching session, one of four days of the year in which futures and options contracts expire at once.

Updated from 8:37 a.m. ET, Sept. 15. 

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