* Yellen raises uncertainty about timing of rate hike
* Ukraine in view as Russia seizes naval bases
* Lennar Corp, Jabil Circuit both rise after results
* Jobless claims rise less than expected
* Futures down: Dow 39 pts, S&P 4.7 pts, Nasdaq 10.25 pts (Adds economic data, Lennar results)
By Ryan Vlastelica
NEW YORK, March 20 (Reuters) - U.S. stock index futures pointed to a modestly lower open on Thursday as investors continued to digest comments from Federal Reserve Chair Janet Yellen, who raised the specter of an earlier-than-expected rate hike.
* Yellen's comments indicated that the first increase in interest rates could come early next year, rather than in the second half of 2015, as had been widely expected by analysts. The comments ended a two-day advance on Wall Street that had taken the S&P 500 near record levels.
* 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.
* Lennar Corp rose 2 percent in premarket trading after the homebuilder reported a sharp jump in its first-quarter profit, helped by higher prices. The results come a day after KB Home posted similarly strong results in a bullish read on the housing market. The iShares U.S. Home Construction ETF edged 0.5 percent higher premarket.
* In a statement released on Wednesday, the U.S. 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. Subsequently, 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, raising questions about the timing of a rate hike.
* S&P 500 futures fell 4.7 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 39 points and Nasdaq 100 futures fell 10.25 points.
* The S&P 500 is within 1 percent of its record closing high, but 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.
* In the latest economic data, jobless claims rose by 5,000 in the latest week, less than expected. At 10 a.m. (1400 GMT), February existing home sales and the March business activity index from the Philadelphia Federal Reserve will be released, with both 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 . Shares rose 2.2 percent to $18.66 before the bell.
* U.S. shares of GlaxoSmithKline fell 1.8 percent to $53.71 in premarket trading after an experimental cancer vaccine failed in a second test, but the British pharmaceutical company said it still hoped to identify a sub-group of patients in which it would work. (Editing by Nick Zieminski)