Nokia, the world's largest phone maker by volume, is aiming to produce a phone running new partner Microsoft's operating system by the end of the year, its chief executive told Reuters.

Clearly there is significant pressure on the teams and the whole company to ensure that we deliver a great Windows Phone product as quickly as possible, and we would certainly prefer to see that in 2011, CEO Stephen Elop said in an interview on the sidelines of the Mobile World Congress trade show in Barcelona.

Elop announced the software tie-up with Microsoft on Friday, but the mainstay of his much-anticipated strategy revamp got the thumbs down from investors, with shares in the Finnish company dropping to their lowest level since 1998.

Because of the pace of software development and advances of Android, and Windows Phone for that matter, and Apple, we had a risk of falling further and further behind because development of our Symbian-based products was slipping, Elop said.

Elop declined to comment on the ongoing process of selling a stake in Nokia Siemens Networks, its telecoms joint venture with Germany's Siemens.

It's a business which very much, strategically, stands on its own, he said.

(Reporting by Tarmo Virki; Editing by Will Waterman)