Companies with female directors outperform those with only men at the top, according to a study of more than 1,000 Asian firms released on Thursday, as calls grow for more diversity in the region's boardrooms.

Firms whose boards were at least 30% female generated more income from their assets than those with all-male boards, found the study by the International Finance Corporation (IFC), the World Bank's investment arm.

"The findings should send a clear message to Asian companies that there is a distinct business case for more women on boards," said Vivek Pathak, IFC's regional director for East Asia and the Pacific.

"Taking the step to have more women on boards is not only good for promoting gender diversity within companies, it also makes smart business sense," he told the Thomson Reuters Foundation in an email.

The IFC researched more than 1,000 companies in China, Indonesia, Malaysia, the Philippines, Singapore, Thailand and Vietnam.

Despite gains in recent years, it said "widespread gender bias" in the region meant women were still viewed as less suitable for senior positions.

Almost two out five Southeast Asian companies had no female board members, its study found.

Thailand is the most gender-diverse, with women holding about 20% of board seats among the listed companies, followed by Indonesia and Vietnam, both at 15%.

"The business case for board gender diversity is strong and relates not just to performance, but also to corporate governance, reputation and fairness," said Risa Rustam, a director at the Indonesia Stock Exchange.

The IFC urged companies to put in place policies to promote more women to senior leadership roles to counter a culture of "old boys' networks".