Tar balls have landed on a Texas beach, meaning oil from the Gulf of Mexico spill has hit all five US Gulf states.

Tests identified the small amount of tar balls found near Galveston as coming from the BP spill.

But officials said it was unclear if the tar balls drifted hundreds of kilometres from the leaking oil well or fell from ships that have worked on cleaning up the spill.

Oil has been leaking from a blown-out sea floor well since 22 April.

The US Coast Guard commander for the Galveston region, Capt Marcus Woodring, said about five gallons (19 litres) of tar balls were found on Saturday on a beach near Galveston and determined to be from the Deepwater Horizon leak.

But he said it was impossible to confirm whether they drifted from the site of the leak or dropped off one of the ships carrying collected oil to Texas to be processed.

The joint BP-US government response team said there were doubts the oil had drifted all the way from the spill.