Worries that China may be about to jettison some of its vast reserves of US Treasury bills depressed the American credit markets yesterday, as China's premier, Wen Jiabao, expressed concerns about their integrity. However, markets globally were threatened by comments from Mr Wen that Beijing is ready to expand its fiscal stimulus if the economic situation worsens.

Mr Wen said: "We have made a huge amount of loans to the United States. Of course, we are concerned about the safety of our assets. To be honest, I'm a little bit worried. I would like to call on the United States to honour its words, stay a credible nation and ensure the safety of Chinese assets."

One of the great "global imbalances" of recent years has been the US-Chinese relationship, America's vast trade deficit being funded by equally lavish purchases by the Chinese of US government paper. Now, as the US government takes up the slack left by the American consumer, the hope in Washington is that China will continue to provide a healthy market for US Treasuries, and in particular for those issued under the $787bn (£563bn) stimulus package passed by Congress.

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About half of China's $2trillion in foreign currency reserves comprise US Treasury bills and other official notes. Mr Wen's remarks came as the G20's finance ministers gathered in London to set the agenda for the main G20 summit, to be attended by the leaders of the world's largest and most dynamic 20 economies, in Downing Street on 2 April.

In London, the president of the World Bank, Robert Zoellick, warned that 2009 would be a "very dangerous year", with "waves of challenges" hitting ministers and central bankers. The fiscal stimulus packages now being planned or implemented by the world's governments need to go further. The danger, he said, was that it would be a case of "too little, too late". He also pointed to the additional danger that many Treasury departments around the world were planning to reverse their stimulus packages in 2010.

He called on governments to follow the International Monetary Fund's guidelines and boost their economies by 2 per cent of GDP through tax cuts and public spending increases. So far, the European Union has committed to only a 1.5 per cent boost.

Again, Mr Zoellick warned that the world's "blameless" poorest nations would be among the hardest hit, and that the World Bank estimated between 200,000 and 400,000 babies will die in poverty each year as a result of the downturn. He pointed to a "fiscal gap" of between $270bn and $700bn that needed to be plugged to rescue emerging economies most at risk, especially some in eastern Europe.

However, splits between the G20 participants are continuing to emerge.

The US Treasury Secretary, Timothy Geithner, is pressing for a new co-ordinated stimulus – but European governments, led by Germany, and the European Central Bank, are reluctant to take on more debt.

The German Finance minister, Peer Steinbrück, said that the world could face a serious inflation problem if it injects too much money into markets to combat the global recession. "We injected a lot of money into the market, a lot of liquidity, we launched fiscal stimulus plans, short-term plans, that at a global level created huge credit growth ... particularly in the United States. We have to confront a process which could quickly present the next problem ... not in the short term, but in the medium term, an inflation problem at a global level," Mr Steinbrück said.

Mr Zoellick, recognising differences between the US and some EU powers over the right response to the crisis, stressed that even if they could agree on the right level of fiscal stimulus, that would not be enough – and that the world's dysfunctional financial system was in desperate need of repair. "Stimulus packages alone are not enough," Mr Zoellick said. "The International Monetary Fund research of some 122 fin-ancial and economic crises shows that turnaround can't happen unless you clean up the bad assets and recapitalise the banks ... If you don't take on the banking issue, the stimulus is just like a sugar high. It pushes some energy into the system but then you get the letdown unless you reopen the credit markets."

The Chancellor, Alistair Darling, who is chairing this weekend's talks, said: "We've got 80 per cent of the world's economy sitting around one table, first this weekend and in a couple of weeks' time in London. We have the opportunity to demonstrate that we not only understand what needs to be done but we actually get on and do it. I think in relation to the banking system the progress in sorting that out is frustratingly slow. What we really need is commitment from people."

Next month's G20 summit, to be hosted by Gordon Brown, seems destined to be bogged down by arguments over virtually every aspect of the economic crisis, from the correct level of fiscal and monetary stimulus through banking regulation and protectionism. The World Trade Organisation is preparing a paper on protectionist abuses by G20 members, which may embarrass some governments. While most observers expect a united front and impressive-sounding words from the eventual communiqué, many will fear that the substance will be lacking.