Nokia said it will continue to sell smartphones using its legacy Symbian software for a long time after its first phones using Microsoft's Windows go on sale.

Symbian is due to be replaced by Microsoft's Windows Phone over the coming two years under a plan announced in February, raising concerns Nokia could drop Symbian phones -- which have lost market share, but generate strong profits -- too early.

"We will be selling (Symbian) devices long after Windows Phone devices from Nokia have already started to appear," Purnima Kochikar, head of developer relations at Nokia, said in an open letter to developers, published late on Friday.

Nokia has promised to introduce its first models using Windows Phone platform at the latest in 2012.

Nokia didn't specify how long it would continue to offer Symbian.

Earlier this week research firm Ovum forecast that Symbian phones will continue on some markets into 2016. Kochikar said coming Symbian models would focus on strong integration of services, as well as have faster processors than so far in the Symbian product line.

"Over the past weeks we have been evaluating our Symbian roadmap and now feel confident we will have a strong portfolio of new products during our transition period -- i.e. 2011 and 2012," Kochikar said.

"They will also include improvements in hardware performance such as GHz+ processing capabilities and faster graphics speeds," she said.

Nokia's current top models, the N8 and the E7, use 680 MHz (megahertz) processors, while rivals top models already use 1 GHz (gigahertz) and faster processors. – Reuters