SEOUL, South Korea -- Samsung Electronics says its January-March operating income has declined 4 percent over a year earlier.

The Asian tech heavyweight said in its Tuesday earnings preview that its operating income for the first three months of this year reached 8.4 trillion won ($8 billion), compared with 8.8 trillion won a year earlier. Sales remained flat at 53 trillion won.

The operating profit was in line with the median estimate by analysts surveyed by FactSet.

The profit decline shows the challenge for Samsung to continue its earnings growth in the face of falling prices of smartphones, its biggest cash cow that brings over two-third of its income.

Samsung, the world's largest maker of smartphones, televisions and memory chips, will announce its full financial results including net income later this month.