HELSINKI, FINLAND: Nokia, the world's top cellphone maker, brought in Microsoft's Stephen Elop to replace embattled chief executive Olli-Pekka Kallasvuo and lead a renewed effort to compete in the smartphone market. The following are key challenges facing Elop:

Ailing high-end phone portfolio

Nokia's long reliance on the Symbian operating system for its smartphones, while profitable given the volume of phones it produces, left it badly exposed when Apple launched its iPhone in 2007.

While Nokia remains the clear leader in terms of cellphone volumes thanks to a strong presence in countries like China, India and Indonesia, its weakness in smartphones has led to Apple becoming the most profitable handset maker.

Nokia was slow to match Apple's touch-screen technology, and has yet to cultivate the wide developer base enjoyed by both Apple and Android developer Google, giving consumers little reason to use its phones.

Kallasvuo disappointed investors in April when he said the roll-out of the latest Symbian software, a key weapon in the fight against Apple, would be delayed until later in 2010, with further upgrades pushed back to 2011.

Nokia has lacked an iconic high-end phone since launching the N95 in 2006, ceding creative ground at exactly the moment when one of the world's leaders in technology design and marketing was set to enter the market.

Implosion in US market

In the late 1990s, when Nokia made its run and passed Motorola to become the top mobile phone maker, a crucial factor was the Finnish firm's success in the U.S. market, both in terms of generating cash as well as consumer buzz.

The buzz has long gone. Now Nokia has well under 10 percent of the U.S. market, lagging rivals including Apple, Samsung and LG. Nokia lacks a presence in the world's leading media market at the moment it needs it most.

When taking the helm of Nokia in 2006 Kallasvuo made one big promise -- he would focus fully on fixing Nokia's problems in the United States, spending a week each month on the problem -- but there has been no improvement since then.

A key stumbling block for Nokia in the U.S. has been the dominance of large operators like AT&T and Verizon, and Nokia's insistence -- some say arrogance -- on designing and selling its phones without operator input.