The Finnish company gave no details about the September event, but Nokia CEO Stephen Elop said earlier on Wednesday the company would unveil soon a new smartphone using Microsoft's latest smartphone software known as Windows Phone 8.

The joint event with Microsoft will occur a week before Apple is expected to unveil its own hotly anticipated iPhone 5.

Nokia has been fighting for survival after ceding vast ground to rivals Apple and Samsung Electronics Co Ltd in recent years. In 2011 it forged a software alliance with Elop's former employer, Microsoft, which also fell behind in smartphone software.

Both Nokia and Microsoft are pinning their hopes on the next Windows-based phone to challenge Apple's iPhone and Samsung's Galaxy smartphone.

Nokia has not commented on specific plans for its Nokia World trade show event in Helsinki on September 5 and 6.

Elop did not deny a September launch when he spoke to reporters earlier on Wednesday. He said only that Windows Phone 8 smartphones would be released in the "relatively near term."

Nokia shares, which have been trading near all-time lows, rose sharply in Helsinki after Elop's comments and gained 3.4 percent to close at 2.0760 euros ($2.55) in Helsinki on Wednesday. Its U.S. shares closed up 6 percent at $2.64 on the New York Stock Exchange.

Also on Wednesday, Standard & Poor's downgraded Nokia's credit rating to BB- with a "negative" outlook.

Elop, who was brought in from Microsoft two years ago to lead Nokia's fight against Apple and Samsung, said he was sticking to his strategy of using Microsoft software despite the limited success of Windows phones so far.

Nokia decided in early 2011 to ditch its home-grown Symbian software in favor of software from Microsoft.

"I don't think about rewinding the clock and thinking about competing elsewhere," he told reporters in Oslo.

"In today's war ... (between) Android, Apple and Windows, we are very clear, we are fighting that with the Windows phone," said Elop, who was in Oslo for a meeting with Telenor ASA Chief Executive Jon Fredrik Baksaas.

Nokia lost 1.53 billion euros in the second quarter and sold just 4 million Windows phones in the period, well short of Apple's sales of 26 million iPhones and Samsung's 50 million smartphones.

Nokia shrugged off S&P's downgrade, saying it had sufficient liquidity and was saving money. S&P cut Nokia's rating in April to BB+ -- junk status -- meaning conservative investors like pension funds will consider it too risky to hold.

($1 = 0.8139 euro)

(Additional reporting by Ritsuko Ando in Helsinki; Editing by Erica Billingham and Matthew Lewis)

• A Permanent Economic Emergency