WASHINGTON -- China always welcomes the United States and Japan to join the Asian Infrastructure Investment Bank (AIIB), and has kept them informed with all the relevant information, China's Finance Minister Lou Jiwei told Xinhua on Saturday.

"The United States and Japan have not been able to join the AIIB due to their own difficulties," Lou told Xinhua on the sidelines of the World Bank-International Monetary Fund (IMF) Spring Meetings, without elaborating.

The United States and Japan decided to stay out of China's initiative by the March 31 deadline as both countries worry that the AIIB may pose a challenge to the Japan-led Asian Development Bank (ADB) and the U.S.-dominated World Bank.

However, China has underlined that the AIIB will take an open and inclusive attitude and be complementary to existing development banks and it will strengthen the region's communication, social and economic development.