* U.S. Fed decides against rate rise
* European, U.S. stocks fall sharply
* Brent set for first weekly gain in three weeks (Updates shares, oil prices)
By Karolin Schaps
LONDON, Sept 18 (Reuters) - Oil prices fell on Friday after the U.S. central bank warned on the health of the global economy and bearish signs persisted that the world's biggest crude producers would keep pumping at high levels.
The Federal Reserve decided against raising interest rates from historic lows on Thursday, saying uncertainty about global economic growth had forced its hand.
The bearish sentiment sent European and U.S. stock markets sharply lower on Friday, with Frankfurt's Dax .GDAXI down more than 3 percent and the U.S. S&P 500 index .SPX down 1.15 percent at 1338 GMT.
The oil market had mixed reactions to the Fed decision, with concerns about economic weakness sending commodities lower, but a fall in the U.S. currency also meant buying dollar-traded crude became cheaper.
"The perception of 'ZIRP (Zero Interest Rate Policy) forever' should provide some underlying support to the commodity complex," said Olivier Jakob, a strategist at Petromatrix, a Swiss-based consultancy.
Brent crude LCOc1 was down 80 cents at $48.28 a barrel at 1340 GMT, after touching an intraday high of $49.75. U.S. West Texas Intermediate (WTI) crude futures CLc1 were trading down $1.55, or 3.3 percent, at $45.35 a barrel.
Brent was set to make its first weekly gain in three weeks, hinting at a turn in momentum for a commodity that has declined nearly 30 percent since spring.
Kuwait, a member of the Organization of the Petroleum Exporting Countries (OPEC), said on Thursday the oil market would balance itself but that this would take time, indicating support for the group's policy of defending market share despite falling prices.
Other sources at OPEC backed this view, saying they expected oil prices to rise by no more than $5 a barrel per year to reach $80 by 2020, with a slowing in rival non-OPEC production growth not enough to absorb the current oil glut.
Iran's deputy oil minister Rokneddin Javadi was quoted as saying the country would unveil new oil contracts in the coming weeks, earlier than previously expected.
The prospect of sanctions-free Iran adding more barrels to an already oversupplied market is fuelling bearish momentum. Javadi also reiterated Iran's plans to regain its oil production share once Western sanctions are removed.
Russia acknowledged for the first time on Friday it would cut production if oil prices fall below $40 a barrel, floating the possibility that one of the biggest oil producers may resort to proactive measures to prop up prices.