Finance chiefs from major powers are to hold emergency talks by telephone to discuss ways to contain the latest turmoil on world financial markets, reports say.
Twin debt crises in the eurozone and the US have caused sharp falls.
Analysts say world leaders want to calm international markets ahead of exchanges reopening on Monday morning.
The rating agency Standard & Poor's on Friday downgraded America's top-notch AAA rating to AA+.
Markets have also been rocked by suggestions that the debt crisis in the eurozone could spread to Italy and Spain.
Sources in Rome quoted by AFP news agency said finance ministers of the G7 nations - the US, Germany, the UK, Japan, France, Canada, and Italy - would hold a telephone conference call to discuss the situation.
Japanese government officials told Reuters it would be "normal to assume" G7 finance chiefs would hold a phone conference before Asian markets open on Monday.
Meanwhile, the European Central Bank (ECB) is to hold a rare emergency meeting on Sunday to decide whether it start buying up the debt of Italy - the latest and biggest economy to be hit by the eurozone crisis.
BBC Business Editor Robert Peston says the decision will be of vital importance, although members of the bank's governing council are said to be divided on the course of action.
After months of wrangling between Democrats and Republicans, the US Congress on Tuesday approved a last-minute deal to raise the US debt limit by up to $2.4tn (£1.5tn) from $14.3tn.
However, it failed to impress Standard & Poor's.
In a statement on Friday its said "political brinksmanship" shown in debates was proof that "the effectiveness, stability, and predictability of American policymaking and political institutions have weakened".
The credit rating downgrade is seen as a major embarrassment for President Obama's administration. It could also raise the cost of US government borrowing.
White House spokesman Jay Carney said on Saturday the deal had been "an important step in the right direction", but that "the path to getting there took too long and was at times too divisive".
He said the US must now "do better".
On Saturday, UK Prime Minister David Cameron - currently on holiday in Italy - discussed the crises with French President Nicolas Sarkozy in a 30-minute telephone call, Mr Sarkozy's office said.
Last week, European Commission President Jose Manuel Barroso said authorities in the eurozone were failing to prevent the sovereign debt crisis from spreading.
That sparked fears that Italy and Spain might become engulfed in the crisis which has led to Greece, the Irish Republic and Portugal already being bailed out.
Both Italy and Spain insist they can service their debt.
But in the past few days the gap between German bonds - the safest in Europe - and Spanish and Italian debt have reached a record since the euro was introduced in 1999.
There have been rumours that the ECB was preparing to buy Spanish and Italian bonds to try to help those countries.