British energy giant BP said on Sunday a small tube was successfully siphoning oil and gas from a ruptured undersea well in the Gulf of Mexico.

BP crews inserted the tube into the well's riser stack on Sunday, after earlier setbacks with a mile-long tube, BP executives told reporters in a conference call.

"It's working as planned and we are very slowly increasing the rate that is coming from the riser tool up to the surface," BP senior executive vice president Kent Wells told reporters at BP's U.S. headquarters in Houston.

"So we do have oil and gas coming to the ship now," he said. But he did not say how much oil was being captured.

The well has been gushing an estimated 210,000 gallons, or 5, 000 barrels, of crude a day into the Gulf of Mexico since the explosion and sinking of the "Deepwater Horizon" drilling rig late last month off the Louisiana coast.

The successful insertion of the 4-inch tube into a 21-inch riser pipe that once connected the rig with an underwater well followed BP's two failed attempts to insert a mile-long tube.

Preparations for an attempt to inject mud into the well to stop the leak permanently were ongoing and would be completed in seven to 10 days, Wells said.