Nearly 200 managers at Samsung Electronics' mobile division have voluntarily returned a quarter of their first-half bonuses, a source with direct knowledge of the matter said, with the firm poised to post its worst quarterly profit in two years.

Samsung earlier this month announced operating profit guidance for April-June that was far below most analysts' forecasts.

The world's biggest smartphone maker is facing slowing market growth and intensifying competition.

The decision to return some of the bonus was partly a gesture to demonstrate that managers are taking responsibility for the earnings decline, and partly a way to show that management will work harder, the source told Reuters, declining to be identified due to the sensitivity of the matter.

Such gestures are not uncommon in South Korea's corporate world, where senior employees of listed companies have been known to take similar steps when performance dips. The source did not give further details, though local media reports estimated that the value of the returned bonuses would likely exceed $2.92m.

A Samsung spokeswoman declined to comment.

Samsung's cash-cow smartphone business has been losing market share as Chinese rivals such as Xiaomi and Lenovo offer increasingly feature-rich phones at lower prices.

Meanwhile, a survey of 35 major global markets by researcher Counterpoint showed the flagship Galaxy S5 smartphone was outsold by Apple Inc's iPhone 5S in the month of May, suggesting that the device was not selling as well as hoped.

Samsung pointed to inventory build up for its mid-to-low tier products as well as the strength of the South Korean won in its earnings guidance last week.

Samsung will disclose its full earnings for April-June on July 31.