A US Grand Prix is still some way off returning to the Formula One calendar despite teams putting pressure on commercial supremo Bernie Ecclestone, Williams founder Frank Williams said.

"It is a disappointment to all the teams that there is no presence in North America," the Briton, whose team's title sponsor is American telecoms giant AT&T, said.

"We pressured Bernie to resolve that but I think his problem as a promoter is that he sees no race track that is good enough, in his standards - that is to say, one that is ready to race on - and he's probably right.

"There are some good layouts in America, really demanding circuits but the safety features don't compare at all to what is required by the (governing) FIA or the GPDA (drivers' association)."

The Canadian GP in Montreal has been dropped from this year's calendar and the US round at Indianapolis made its last appearance in 2007.

Despite slumping sales in the global credit crunch, the United States is a key market for car manufacturers that dominate the sport, such as Mercedes and BMW, as well as for many of the sponsors.

The question of returning to America has been back in the spotlight this week after plans were revealed for a US-based team to enter the championship in 2010.