SEOUL (BLOOMBERG) - The fledgling Asian Infrastructure Investment Bank's membership will increase by about a fifth to 70 "soon", the president-designate of the China-backed bank said.

"We do have quite a number of countries waiting outside to join," said Mr Jin Liqun, speaking in Seoul on his first overseas visit since becoming the bank's president-designate.

"I'd expect very soon we will have more than 70 countries." Mr Jin declined to name any of the potential members that may join the 57 founders.

China is creating the first multilateral investment bank in a generation to finance infrastructure projects, frustrated that it only has a small say in US- and Western European-dominated financial institutions such as the World Bank and the International Monetary Fund.

The US and its Asian ally, Japan, declined to join the AIIB, raising concerns about transparency and governance. Even so, countries including Britain, Germany and Australia signed up.

"The door is kept open for Japan and the US to become members of the AIIB, and I think we will continue to have dialogue," he said.

The bank will focus on quality, not the quantity of projects, and will promote the "green economy," Mr Jin said. The AIIB will start to fund projects in early 2016. North Korea is a country that the AIIB would like to help, Mr Jin said.

China is the biggest AIIB shareholder with an initial capital subscription of US$29.8 billion and 26.06 per cent of its voting rights, according to the articles of agreement. India, Russia, Germany and South Korea round out the top five.