KUALA LUMPUR—Malaysia's top two banks said Tuesday they plan to start merger talks with the country's No. 5 lender, RHB Capital Bhd., in a bid become one of Southeast Asia's largest banking groups.

Malayan Banking Bhd., the country's largest lender by assets, and CIMB Group said in separate statements they had received the central bank's nod to commence negotiations with RHB Capital and its shareholders. The Bank Negara Malaysia's approvals—a prerequisite for any corporate negotiations involving local financial institutions—are valid for three months.

Talk of banking mergers has been rife after RHB Capital's 25% shareholder, Abu Dhabi Commercial Bank, put its stake up for sale.

Statements by both Maybank and CIMB Group said the Bank Negara's approval allows them to start talks with "RHB Capital and its major shareholders." Officials from both Malaysian banks declined to comment when asked if this could involve bidding for the Abu Dhabi stake.

"The potential transaction, if it materializes, would be consistent with Maybank's vision to become a regional financial services leader, and support its strategic objectives of being the undisputed No. 1 retail financial services provider in Malaysia and the leading Islamic bank in Asean [Association of Southeast Asian Nations]," Maybank Chief Executive Wahid Omar said in a statement.

The transaction will be evaluated based on the potential for value creation for Maybank's shareholders, he added.

CIMB Group Chief Executive Nazir Razak said: "It is incumbent on us to engage on this opportunity to put forward a value-creating merger between the two banks and support the national banking consolidation agenda."

Mr. Nazir, however, said negotiations haven't yet commenced, and it may take "a few weeks and possibly more, for us to explore possibilities with RHB's management and shareholders."

Analysts said the completion of the acquisition by Hong Leong Bank Bhd. of the assets and liabilities of EON Capital Bhd. to create the country's No. 4 bank this month was one likely trigger for Tuesday's move.

"We see the acquisitions as primarily [for the banks] to gain size," said ECM Libra head of research Bernard Ching.

OSK head of research Chris Eng said he doesn't see immediate benefits for either party. "We're a bit surprised by this [move]," he said, adding that CIMB had earlier dismissed the market rumor that it was planning to bid for RHB while Maybank had said it plans to focus on "organic growth."

Mr. Eng said with a likely bidding war for RHB, the situation "could get interesting."