Finance ministers and central bankers of the Group of Seven major industrialized nations will confer by telephone on turmoil in the financial markets later on Saturday or on Sunday, a senior European diplomatic source said.

The source said Friday's unprecedented downgrading of the United States' credit rating by Standard & Poor's had added a global dimension on top of the euro zone's debt crisis, raising the need for international coordination.

"The G7 will confer by telephone. It's not yet confirmed whether it will be in one stage or in two stages, tonight and tomorrow," the source said.

French Finance Minister Francois Baroin, who would chair such a meeting under the French presidency of the G7 and G20, said in a radio interview it was too early to say whether there would be an early G7 meeting.

G7 finance ministers and central bankers were due to meet in early September in the French city of Marseille.

A British government source said a conference call between senior financial officials from the G7 was taking place on Saturday but said no draft document had yet been produced.

"I don't think there's any document being worked through at the moment," the source said.

(Reporting by Paul Taylor and Michael Holden in London; Editing by Sophie Hares)