Japan remains ambiguous over China-proposed AIIB

"Joining negotiation will increase the amount of information available and the final decision on whether to join the bank will be easier to make," said Seguchi, also an expert on Chinese economy, who suggested that Japan should directly involve in the AIIB so as to seek internal reform, rather than staying out and asking for reform.

Recent reports said that Japan and China would like to discuss the AIIB in June as the two sides are eyeing finance ministers meeting in June in Beijing.

International lenders have expressed their willingness to cooperate with China's AIIB initiative.

"We need to work together, we need to cooperation," Christine Lagarde, managing director of the International Monetary Fund, said Thursday in a speech in Washington.

She hailed the AIIB as a "great initiative" and said the trend reflects the transformation of the global economy with new key players.

For the US part, Treasure Secretary Jacob Lew softened his hostile stance toward the AIIB, saying last month after his two- day Beijing visit that his country "stands ready to welcome new additions to the international development architecture, including the AIIB," adding that "these institutions complement existing international financial institutions."

"I was encouraged by my conversations in Beijing in which China 's leaders made clear that they aspire to meet high standards and welcome partnership," Lew said, adding that having the AIIB co- finance projects with existing institutions will help demonstrate a commitment to the highest standards of governance, environmental and social safeguards and debt-sustainability.

The AIIB, with an expected initial subscribed capital of $50 billion, will be an international financial institution to fund infrastructure projects in Asia and is expected to be formally established by the end of this year.