However, US President Barack Obama said that there was more work to do before the gusher was permanently stopped.
BP was conducting a pressure test after it choked off the well on Thursday.
"We've seen no negative evidence of any breaching there," said Kent Wells, BP's senior vice-president of exploration and production.
Speaking at the White House, Obama cautiously welcomed the news.
"We won't be done until we actually know that we've killed the well and that we have a permanent solution in place. We're moving in that direction, but I don't want us to get too far ahead of ourselves."
BP's shares rose in London on Friday on hopes that it has at last been able to stop the oil that has been leaking into the Gulf of Mexico for the past three months and can focus on the clean-up.