Wall Street gained, while oil prices fell on a government report showing a buildup in US crude stockpiles.

Wall Street moved higher as investors felt reassured by former FBI Director James Comey's written statement for the record, released before he appears before the Senate Select Committee on Intelligence on Capitol Hill on Thursday.

"They were hoping that there wasn't going to be anything in there that was more inflammatory," Peter Costa, president of trading firm Empire Executions, told Reuters. "The testimony wasn't as disastrous as it could have been," Cost said of the prepared remarks, adding that the market was relieved no damaging details emerged and his testimony "more than likely isn't going to blow up into some big fiasco, another thing that the president has to deal with."

In 3.15pm trading in New York, the Dow Jones Industrial Average rose 0.15 per cent, while the Nasdaq Composite Index gained 0.25 per cent. In 3pm trading, the Standard & Poor's 500 Index advanced 0.15 per cent.

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The Dow moved higher as gains in shares of Nike and those of American Express, recently both up 1.5 per cent, outweighed slides in shares of Caterpillar and those of General Electric, recently down 1.2 per cent and 0.9 per cent respectively.

Meanwhile, Bill Gross, manager of the US$2 billion Janus Henderson Global Unconstrained Bond Fund, said US markets are at their highest risk levels since before the 2008 financial crisis because investors are paying a high price for the chances they're taking, Bloomberg reported.

"Instead of buying low and selling high, you're buying high and crossing your fingers," Gross said Wednesday at the Bloomberg Invest New York summit.

Oil prices dropped, sliding more than 4 per cent after an Energy Information Administration report showed US crude inventories unexpectedly rose last week and fuelled concern about the global glut. West Texas Intermediate crude oil dropped 4.8 per cent to US$45.87 a barrel in New York.

"This is really unexpected," Gene McGillian, market research manager at Tradition Energy in Stamford, Connecticut, told Bloomberg. "It really looks as if fears of oversupply is what's driving the market."

In Europe, the Stoxx 600 Index finished the day with a 0.1 per cent decline from the previous close. France's CAC40 Index also slipped 0.1 per cent, and Germany's DAX Index eased 0.1 per cent as well.

The UK's FTSE 100 Index dropped 0.6 per cent.

UK voters head to the polls on Thursday, without guarantees Prime Minister Theresa May will secure another majority.

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"If the Conservative party extends its majority, markets will be pretty calm, but anything less than that is going to have people worried about how we approach the Brexit negotiation," Luke Hickmore, senior investment manager at Aberdeen Asset Management, told Reuters.