There are an estimated 45 million users of Twitter

Social networking website Twitter has confirmed that it has closed a "significant round of funding".

Co-founder Evan Williams said in a blog post that the site had secured money from five investment firms.

However, he did not confirm earlier reports that suggested the firm had managed to secure $100 million (£62m), which would value the firm at $1bn.

The site, which allows users to write and share 140-character messages, has more than 45 million users worldwide.

The site had previously raised $35m in February in a deal that valued the business then at $255m.

"It was important to us that we find investment partners who share our vision for building a company of enduring value," wrote Mr Williams.

"Twitter's journey has just begun."

Industry watchers have pointed out that the firm still has no way of making money.

However, earlier this month the site revised its terms and conditions to allow advertising on its service.

"We leave the door open for advertising. We'd like to keep our options open," wrote co-founder Biz Stone in a blog.

The new funding has come from new investors Insight Venture Partners and T Rowe Price, as well as existing backers Institutional Venture Partners, Spark Capital and Benchmark Capital.