* Ukraine in view as Russia seizes naval bases
* Jabil Circuit outlook tops expectations
* Jobless claims, housing data both on tap
* Futures down: Dow 45 pts, S&P 5.6 pts, Nasdaq 11.5 pts
NEW YORK, March 20 U.S. stock index futures were lower on Thursday as investors continued to digest recent comments from Federal Reserve Chair Janet Yellen, who raised the specter of an earlier-than-expected rate hike.
* Wall Street fell on Wednesday, snapping a two-day advance that had taken the S&P 500 near record levels, after the central bank made clear that it would rely on a wide range of measures in deciding when to raise interest rates, dropping the unemployment rate as its definitive metric.
* Yellen said the "considerable period" between the end of the Fed's bond-buying stimulus program and the first rate increase from the central bank could be six months. Many analysts had been expecting the first increase to come in the second half of 2015.
* Geopolitical concerns also continued to weigh on sentiment as Russian troops seized two Ukrainian naval bases, including a headquarters in the Crimean port of Sevastopol. The United States warned Russia that it was on a "dark path" to isolation in the biggest crisis between the two countries since the Cold War.
* While few U.S. companies have outsized exposure to the region, investors are concerned about the fallout to any escalation in tension.
* S&P 500 futures fell 5.6 points and were below fair value, a formula that evaluates pricing by taking into account interest rates, dividends and time to expiration on the contract. Dow Jones industrial average futures fell 45 points and Nasdaq 100 futures fell 11.5 points.
* The S&P 500 is within 1 percent of its record closing high, though volume has been anemic on positive market days, suggesting limited conviction behind the move.
* Volume is expected to surge on Friday as options expiration takes place alongside multiple index rebalances. Credit Suisse estimates $14 billion in gross trading will stem from the S&P 500 index rebalance, with another $6 billion coming from rebalancing in other indexes.
* Investors are also looking ahead to the latest economic data, with weekly jobless claims on tap for release at 8:30 a.m. (1230 GMT) Claims are seen rising by 10,000 to 325,000.
* February existing home sales and the March business activity index from the Philadelphia Federal Reserve are both scheduled for release at 10 a.m. Both are seen rebounding from weakness in the previous month.
* Jabil Circuit Inc forecast 2015 core earnings above Wall Street estimates as the struggling contract electronics maker expects to recover from the loss of its business with BlackBerry Ltd .
* Deutsche Bank is on track to meet its 2015 investment banking targets, it said after restating its 2013 results to include around 350 million euros ($487 million) of settlement costs and around 50 million more to cover impaired assets. U.S. shares of the bank fell in light premarket trading.
* U.S. shares of GlaxoSmithKline fell 1.7 percent to $53.78 in premarket trading after an experimental cancer vaccine failed in a second test, though the British pharmaceutical company said it still hoped to identify a sub-group of patients in which it would work.