BP Senior Vice President Kent Wells said the amount being drawn was gradually increasing, and it would take several days to measure it.
The tube was carefully placed into the 21-inch piping at the sea floor by engineers gingerly steering deep-sea robots.
The company has spent three weeks trying to contain the oil leak that's been fouling the Gulf of Mexico, and the latest effort had several setbacks.
Even as the company reported the success, scientists warned oil that has already spewed into the gulf could have dire consequences for the environment.
A researcher tells The Associated Press that computer models show the black ooze may have already entered a major current flowing toward the Florida Keys.