U.S. stock index futures pointed to a modestly lower open on Friday as crude oilprices continued to fall, putting the S&P 500 on track for its third straight weekly decline.

Markets have been volatile this week, with the S&P posting both its biggest one-day gain since early February and its biggest one-day loss since early January.

Crude oil CLc1 fell 2 percent, after the International Energy Agency said a global oil glut was building and U.S. oil production showed no signs of slowing. The commodity has fallen in six of the past seven sessions and could continue to weigh on energy names.

Equities have recently been driven by the U.S. dollar, with the S&P having a high inverse correlation to the currency. Thursday's equity rally corresponded with the biggest one-day drop in the U.S. dollar index .DXY in a month.

The dollar index has risen almost 2 percent this week, and is on track for a fourth week of gains. Investors see the continued strength in the greenback as a threat to multinational corporate profits. The index rose 0.15 percent on Friday.

Wall Street has also been focused on when the Federal Reserve will raise interest rates, with some strong economic data recently suggesting the first hike could come as early as June. Higher rates tend to raise borrowing costs for companies and individuals and crimp spending, though strong indicators are seen as better for the market in the long term.

Trading could be volatile ahead of next week's Fed meeting, when the central bank could provide further insight into when the first rate increase will come.

FXCM Inc (FXCM.N) jumped 15 percent to $2.48 in heavy premarket trading a day after the company reported fourth-quarter earnings that beat expectations. This was the currency broker's first quarterly report since the removal of the cap on the Swiss franc sparked massive losses that pushed it to take a rescue loan.

The Dow is on track for a weekly rise of 0.2 percent while the Nasdaq is on track for a drop of 0.7 percent. The S&P is down 0.3 percent on the week, and if the benchmark index ends negative for the week, that will mark its third straight weekly decline. Still, Thursday's rally lifted the Dow and S&P back into positive territory for 2015, and the S&P is 2.4 percent from its record closing high.

Futures snapshot at 8:57 a.m. EDT:

* S&P 500 e-minis ESc1 were down 3 points, or 0.15 percent, with 162,782 contracts changing hands.

* Nasdaq 100 e-minis NQc1 were down 1.75 point, or 0.04 percent, in volume of 15,078 contracts.

* Dow e-minis 1YMc1 were down 32 points, or 0.18 percent, with 11,445 contracts changing hands.