More than 20 countries are waiting to join the China-initiated Asian Infrastructure Investment Bank (AIIB), its designated president said Saturday as he allayed concerns it was aimed to challenge US and Japanese influence in the region.

The number could bring the AIIB on a par with or even surpass membership in the Japan-led Asian Development Bank (ADB), which currently has 67 members, 19 of them outside of the Asia Pacific, according to its website.

"We have 57 countries (which are potential founding members) and to my knowledge...more than 20 countries are on the waiting list," AIIB president-designate Jin Liqun told delegates of the Singapore Summit conference, without naming them.

"That makes it more than 70. I'm sure there will be more countries which will be interested," added Jin, a former Chinese vice finance minister who was put forward by Beijing to head the bank and was elected in August by the prospective founding members.

The AIIB has been viewed by some as a rival to the Washington-based World Bank and the Manila-based ADB, which has been headed by Japan, its biggest donor, since it was founded in 1966.

The US and Japan -- the world's largest and third-largest economies, respectively -- have notably declined to join the AIIB although they are members of the ADB, but Jin said the door is open to both.

"It takes longer for some countries to reach their internal consensus in making their decision," he said.

"We've been very much patient. As you know, the door is open to all of the countries and if they make a decision just pick up the phone, make a call and we can handle the rest of the business," he said, referring to the US and Japan.

Of the 57 AIIB potential founding members, 50 have already signed, including Australia, while the seven others have until the end of the year to affix their signatures.

The bank will be based in Beijing and will have a capital of $100 billion, with $20 billion paid initially. It is expected to start operations next year.

China will be the biggest shareholder with 30 percent. Among non-Asian participants, Germany is the largest shareholder with 4.5 percent, followed by France with 3.4 percent and Brazil on 3.2 percent.

Jin said the AIIB is not aimed at challenging the ADB and the World Bank for influence in Asia as the region's funding requirements are so massive.

He cited ADB estimates that Asia's infrastructure demand could reach $730 billion per year by 2020, vastly exceeding available resources from existing multilateral lenders.

"Creating AIIB has never been about challenging existing institutions. The world stage is spacious enough to allow many actors to perform," Jin said.