The China-led Asia Infrastructure Investment Bank has 47 countries that have joined or applied to be part of it, as US allies reject Washington's concerns, Zhao Yinan reports from Beijing.

Washington has appeared to soften its stance on the new Asian Infrastructure Investment Bank, which some view as a challenger to the US-dominated World Bank, after seeing many key allies flock to join the China-led project.

US Treasury Secretary Jacob Lew said on Monday that the US is looking forward to cooperating with the financial institution, which was first proposed by Beijing in 2013 and aims to boost funding for infrastructure construction in Asia's developing nations.

Previous reports quoting White House insiders suggested the US had been angered by allies such as Britain rushing to sign up with the AIIB.

Yet, analysts say the tone of Lew's comments, which came during his visit to Beijing as the special envoy of US President Barack Obama and just a day before the deadline for applications for founding members of the bank, leaves the door open for the US to adopt an observer status in the coming months when details of the bank are settled.

The change in the US stance may have to do with Japan, its strongest ally in Asia, announcing it is likely to join the AIIB in a few months, according to a Financial Times report that quoted Masato Kitera, the Japanese ambassador to Beijing.

In a meeting with Chinese Premier Li Keqiang on Monday, Lew said the US welcomes and supports proposals that are helpful to infrastructure construction, and cooperation on that regard can be carried out through any mechanisms accepted by the two countries, including the annual China-US Strategic and Economic Dialogue, the World Bank and the AIIB.