LONDON - Foreign policy analysts said US Vice-President Joe Biden's tour of Asia had two key aims - reassuring China that its massive holding of US debt was safe and intensifying Washington's role in Asia.

Biden arrived in Japan on Tuesday for the final leg of his Asian tour, which included six days in China and a one-day visit to Mongolia.

His tour came at a difficult time for the US economy and the US wanted to highlight Beijing and Washington's shared interests, Michael Cox, a professor at London School of Economics and Political Science, told China Daily.

China is the largest foreign holder of US debt, and Biden used the official visit - his first as vice-president - to assure its leaders that their massive investment remained safe after an unprecedented US credit downgrade.

Kerry Brown, senior fellow at London-based Chatham House, said: Biden's visit aimed to "develop a narrative about the positive sides of the relationship between the US and China".

Even in the most difficult areas, the US and China have got to talk, Brown said.

These are testing times for China-US relations, their respective economies and for the global economy, said Rosemary Foot, a professor of international relations at St Antony's College, Oxford University.

"Strong economic and political relations will help build cooperative ties between the two societies and help to reduce conflict and tension in other areas of the relationship," Foot said.

Following visit to China, Biden visited Mongolia and Japan, highlighting Washington's role in the region.

Biden made a one-day stop in coal-rich Mongolia on Monday, where he praised the growing diplomatic relationship between the US and Mongolia.

In Japan, he expressed steadfast US support for its close ally in the wake of the recent earthquake, tsunami, and nuclear disaster.

Cox said Biden's visit to Japan - Washington's closest ally in the region - was necessary, otherwise it could be implied that China is more important to the US than Japan. "Japan needs reassurance about its importance - as well as China - in terms of its importance in US relations with the Asia-Pacific region."