"As we draw up our plans, we must accept that the biggest risk to recovery would be to exit before the recovery is real," he added.
The chancellor, who is hosting the talks, has now been joined by Prime Minister Gordon Brown at St Andrews.
John Hilary, executive director of anti-poverty charity War on Want, said the G20 governments had not done enough to address the root causes of the economic crisis and to prevent it happening again.
Meanwhile, the International Monetary Fund has voiced concerns about government finances.
It said government debt in the developed G20 countries is likely to reach 118% of annual national income (GDP) in 2014.
It will take years of spending cuts and higher taxes to get debts down to what the IMF calls safe levels.
Once again, France is continuing to press for more to be done to stop excessive bonuses in the banking sector.
"I would hope that a little bit more can be done, to actually cast in stone the fact that we want to stop excesses, stop abuses, and bonuses that are strictly risk incentives," France's Finance Minister Christine Lagarde told the BBC.
Many say the huge bonuses on offer are to blame for bankers taking the kind of risks that led to the financial crisis.
On climate change, several nations want a discussion under the banner of the G20, wary that signs are not looking good ahead of December's climate summit in Copenhagen, where nations hope to thrash out a successor to the Kyoto treaty.
Alistair Darling on the aims of the G20 summit
Mr Darling said on Saturday that there needed to be real progress.
"Finance ministers must be engaged because if there isn't an agreement on contributions the Copenhagen agreement is going to be much more difficult."
The UK government said this month it was highly unlikely that a new legally binding climate treaty could be agreed this year - and said that a full treaty may be a year away.
Several countries appear to be waiting for the US position to become clear.
But, with healthcare a domestic priority and various climate bills making their way slowly through Congress, Treasury Secretary Timothy Geithner has not been in a rush to make promises about what the Obama administration can and cannot do.
Many have voiced concerns about the weakness of the US dollar and strength of the euro and yen, our correspondent said, although G20 ministers did not specifically discuss currencies.
Some countries also remain worried about the rate of progress over closing down tax havens, a key pledge made at the London G20 summit earlier this year.
This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.