China is discussing ways in which a new regional lender being pushed by Beijing can cooperate with the Asian Development Bank (ADB), Finance Minister Lou Jiwei said on Sunday, as he sought to fend off concerns that the two banks would become rivals.

Some 27 countries have now signed up to participate in the Asian Infrastructure Investment Bank (AIIB), a $50 billion fund slated to begin operations at the end of the year providing project loans to developing countries.

AIIB is being seen by some as a challenge to the Manila-based ADB and World Bank, however, with the United States urging countries to think twice before signing up.

Lou described the relationship between AIIB and the ADB as "complementary" and said he had held discussions earlier on Sunday with ADB president Takehiko Nakao on how the two could cooperate.

"We discussed what safeguard standards we should adopt. I don't agree (with suggestions) which one is the best," Lou said during a conference in Beijing.

The United States, worried about China's growing diplomatic clout, has questioned whether the AIIB will have sufficient standards of governance and environmental and social safeguards.

Lou said the bank would reference the best practices of multilateral agencies but added that some of those measures could be cumbersome.

"We don't believe some bureaucratic or complex ways are good," he said.

He said the AIIB was dominated by developing countries and their requests and demands needed to be respected.

Early opposition to the AIIB from Western countries partially dissolved after Britain said this month it would join, with France, Germany and Italy swiftly following suit.

Lou told China National Radio on Saturday 27 countries have now signed up.

"Their participation shows our openness and, with some rich countries joining, this will help improve the Asian investment bank's credit rating and reduce costs," he said.

Japan, Australia and South Korea, all major U.S. allies, remain absent from the AIIB's list of members. A Japanese government spokesman said on Friday Tokyo maintained a "cautious position" on participation.

Confirming the talks with Lou, ADB president Nakao said his bank was ready to cooperate with the AIIB but no decisions had been made on specific areas.

"We have already started sharing knowledge regarding procurement systems (and other technical matters)," he told Reuters on the sidelines of the conference.

Nakao earlier said Asia had a huge need for infrastructure financing, reiterating comments made by Lou.