GLOBAL MARKETS-Shares rise on optimism before U.S. jobs data; oil tumbles

* Drop in U.S. jobless claims fuels optimism

* Traders eye U.S. March nonfarm payrolls data

* Oil slips as Iran talks back in focus

* U.S. dollar falls ahead of U.S. jobs data (Adds close of European markets, updates prices)

By Sam Forgione

NEW YORK, April 2 (Reuters) - U.S. stocks snapped back on Thursday after two sessions of losses, as a fall in jobless claims boosted optimism ahead of a key payrolls report, while oil tumbled on fears of worsening oversupply if sanctions over Iran's nuclear program are dropped.

Data showing an unexpected fall in the number of Americans filing new claims for unemployment benefits boosted sentiment before the U.S. Labor Department's nonfarm payrolls report for March, which will be released on the Good Friday holiday.

U.S. and European stock markets will be closed on Friday. Economists polled by Reuters expect a gain of 245,000 jobs, down from 295,000 in February.

Trading could become volatile as participants position for Friday's data. Weak payrolls would not necessarily spook Wall Street, however, since a below-consensus jobs number could ease concerns of a nearer-term Federal Reserve interest rate rise.

"People are looking ahead to the jobs report and right now it would be a surprise if it was weaker than expected," said John Carey, portfolio manager at Pioneer Investment Management in Boston, though he said playing it ahead of time could be tricky.

Oil prices fell as much as 4 percent after global powers negotiating a nuclear deal with Iran called for a news conference after more than a week of talks in Switzerland. Tehran is hoping for a deal that will end crippling economic sanctions and allow it to sell millions of barrels of oil.

A measure of equity indexes worldwide rose, boosted by gains in U.S., emerging markets and Japanese shares. European shares slipped after Greece's lenders said the country needed to do more to unlock financial aid.

MSCI (NYSE: MSCI - news) 's all-country world index was last up 0.75 percent, at 427.74 while its emerging markets index gained 1.32 percent to 995.88. The FTSEurofirst index of 300 leading European companies closed down 0.20 percent at 1,586.76.

The Dow Jones industrial average was last up 0.21 percent, at 17,735.54. The S&P 500 was up 0.28 percent, at 2,065.38. The Nasdaq Composite was up 0.08 percent, at 4,884.03.

Brent crude was last down 4.4 percent, at $54.58 a barrel. U.S. crude was down 2.9 percent, at $48.63.

Prices of safe-haven U.S. Treasury debt slipped. Benchmark 10-year Treasury notes were last down 7/32 in price to yield 1.89 percent, from a yield of 1.87 percent late Wednesday.

The dollar slipped against major currencies, despite the strong U.S. data, as traders took profits from bullish dollar positions ahead of the jobs report, which could lead to volatile trading on thin volumes in the currency market.

Thursday's unemployment data, "after disappointments, will help the dollar, but the focus is still on Friday's jobs report," said Vassili Serebriakov, currency strategist at BNP (Paris: FR0000131104 - news) Paribas in New York.

The dollar index, which measures the greenback against a basket of six major currencies, was down 0.74 percent at 97.458. Spot gold fell $3.75 or 0.31 percent, to $1,200.25 an ounce. (Reporting by Sam Forgione; Additional reporting by Lionel Laurent in London and Daniel Bases and Ryan Vlastelica in New (KOSDAQ: 160550.KQ - news) York; Editing by James Dalgleish)