Any changes in the makeup of China's foreign exchange reserves would have to be very slow, International Monetary Fund chief economist Olivier Blanchard said on Friday.

Blanchard's remark to a Hong Kong business lunch came one day after China's finance ministry announced that its investment in Japanese government bonds in May was nearly triple the annual amount it has invested previously.

Chinese investors had bought a net 735.2 billion yen ($8.3 billion) in Japanese government bonds in May, more than the 541 billion yen purchased in the four months previously, the ministry said.

The huge increase sparked questions about whether China was moving to shrink its exposure to US government debt in favour of relatively stable Japanese government bonds.

China has sought to diversify its vast investments away from the US dollar and Europe since the onset of the global financial crisis.

Blanchard said the IMF welcomed China's recent move to let its yuan currency trade more freely, although he added that any rise was unlikely to be rapid.

The move followed international calls for Beijing let the currency's value rise over concerns that it unfairly made Chinese exports cheaper.