Prime Minister David Cameron has spoken to French President Nicolas Sarkozy to discuss eurozone finances and the US credit rating being downgraded.
Downing Street said that, in a phone conversation, they agreed on "the importance of working together".
Meanwhile, Chancellor George Osborne, who is on holiday, spoke to the head of the International Monetary Fund and the current European Union president.
But Labour accused the government of "complacency" in the financial crisis.
On Friday, Standard & Poor's downgraded the AAA rating for the US for the first time, citing budget deficit concerns.
On the same day, European stock markets continued to fall with investors worried about the eurozone debt crisis and the weak US economy.
It came as fears over the ability of governments to pay their debts - which have led to Greece, the Irish Republic and Portugal already being bailed out - spread to Spain and Italy.
Correspondents say the downgrade could erode investors' confidence in the world's largest economy.
Meanwhile, shadow chancellor Ed Balls criticised the prime minister, chancellor and their deputies for taking their holidays at the same time.
Despite being on holiday, Mr Cameron discussed "the euro area and the US debt down grade", according to a Downing Street spokesman.
"Both agreed the importance of working together, monitoring the situation closely and keeping in contact over the coming days," he said.
Meanwhile, the chancellor, who is currently on holiday in the US, spoke to Jacek Rostowski, finance minister of Poland, which currently holds the rotating presidency of the European Union.
Mr Osborne also spoke to Christine Lagarde, the new head of the International Monetary Fund.
A Treasury spokesman said the phone conversations were part of ongoing international discussions about the impact of the US credit downgrade on the global economy and problems in the euro area.
But shadow chancellor Ed Balls launched an attack on the government's handling of the current financial crisis.