U.S. oil fell for a fourth day on Tuesday, swept lower by concern over next week's vote on Britain's possible European Union exit, which overshadowed signs of a return to health for crude prices.
Safe-haven German Bunds yields fell below zero for the first time, while industrial commodities and equity markets, seen as more vulnerable to economic risk, dropped after polls showed Britain's "leave" campaign leading before a referendum on EU membership.
This overshadowed a more upbeat forecast for oil demand growth from the International Energy Agency, which said the oil market is essentially balanced after two years of surpluses.
Brent crude oil futures were down 57 cents, or 1.1 percent, to $49.78 a barrel. U.S. crude futures settled 0.8 percent lower, or 39 cents, at $48.49 a barrel.
"Even as both OPEC and the International Energy Agency talk about a tighter oil market, fear of the fallout from a UK exit from the euro zone is throwing global markets into a tizzy," said Phil Flynn, an analyst at Price Futures Group in Chicago.
The campaign for Britain to leave the EU has a "significant lead" ahead of the referendum, according to a poll from TNS. Some 47 percent of likely voters said they will opt to leave the EU, compared with 40 percent who want to stay, according to the poll of 2,497 people.
"This (the referendum) has rattled a lot of financial and commodity trader/investors with money seemingly starting to flow to the so-called safe-haven U.S. dollar until the dust settles and the voting is concluded," said Dominick Chirichella, senior partner at the Energy Management Institute in New York.
"The strong U.S. dollar versus most currency pairs is a negative price directional driver for the oil complex."
If Britain voted to leave the EU, a prospect dubbed "Brexit", investors fear the bloc could slip into recession, which in turn could undermine oil demand.
The dollar has risen about 1.5 percent from its June lows against a basket of currencies, spurred by Brexit fears.
Reflecting this extreme nervousness, volatility in the pound spiked to its highest in at least 20 years, rising even beyond the heights seen when U.S. investment bank Lehman Brothers collapsed in late 2008.
Adding to the uncertainty in the markets was the ongoing two-day U.S. central bank meeting.
"Not increasing interest rates would be interpreted as the U.S. economy is not healthy enough for increasing interest rates," a U.S. trader said.
Concerns about Chinese growth are also weighing on sentiment, enough to set aside bullish signs such as a U.S. government forecast on Monday that shale oil output is expected to fall in July for the seventh consecutive month.
