'There is strong interest in the yuan as a reserve currency. Investing in yuan is something that the central banks are considering,' said Daniel Navia, director for central banks and sovereign wealth funds at BBVA, Spain's second-largest bank.

Navia declined to name the central banks that he said were considering such moves, as some were BBVA's clients and foreign exchange reserves remained a sensitive and confidential issue in many countries.

Earlier this month the People's Bank of China, mainland China's central bank, said it had extended its yuan-denominated cross-border trade settlement scheme to cover all domestic enterprises with import-export licences, in an effort to promote the settlement of more foreign trade transactions in yuan, rather than in US dollars or euros.

Navia, a former Spanish central banker, told the South China Morning Post that Beijing was smart to promote the yuan as a trade currency, since this would make global central banks consider holding part of their foreign exchange reserves in yuan.

Meanwhile, there were also growing concerns among government officials and market participants that the weighting of US dollar assets in central bank portfolios might be on the decline.

Late last year, Nigeria's central bank governor, Lamido Sanusi, said the country planned to diversify its US$33 billion in foreign exchange reserves away from US dollars by switching about one-tenth of the stockpile into yuan, though the central bank also said US dollars would remain an important part of the holdings.

Due to a limited supply of yuan assets in global markets, Navia said it would still be at least five years before central banks, particularly in Latin American and African countries where resources-related trade with China grew fast, made yuan a substantial part in their reserves.

'Right now, we are in the early stage of yuan internationalisation. It would be difficult for a central bank to have a large exposure. I think five years would be the minimum for this to happen,' he said.

The head of mainland China's central bank, Zhou Xiaochuan, said during the top annual political summit in Beijing last week that the currency was getting closer to its fair value against foreign currencies.

Some analysts took the comment as a signal to reject international calls, in particular from some US politicians in Washington, for Beijing to accelerate the appreciation of its currency.

But to those central banks that want to keep yuan as part of their reserves, the appreciation of the currency - whether fast or not - may not be the top concern.

'Expectations of the yuan's appreciation are not as important for central banks as they are for market participants, because their investment objectives are different,' said Navia. 'The central bank's priority, rather, is to have a diversified reserve in a basket of currencies.'

Most central banks contacted by BBVA also believed mainland China's macroeconomic situation was more important than the market valuation of the currency, he added.

$3.3b

How much yuan, in US dollars, Nigeria's central bank plans to purchase for its foreign exchange reserves