Google's artificial intelligence (AI) program on Tuesday beat South Korean professional Lee Sedol in the ancient board game Go, recovering from Sunday's loss to win its fourth match in a five-game series. AlphaGo, developed by Google subsidiary DeepMind, made history last year by becoming the first machine to beat a human pro player, but Lee, one of the world's top players, was seen as a much more formidable opponent.

"I am disappointed that the matches are over, and also disappointed that I could not end the series on a high note," Lee told reporters, thanking fans for their support and adding that he had wholeheartedly enjoyed the games.

"I think the humans still have a good chance," he added.

Go involves two contestants moving black and white stones across a square grid, aiming to seize the most territory. The game offers too many possible moves for a machine to win by brute-force calculations only, so artificial intelligence is required in order to score a win against a human opponent.

Google aims to apply the technology underlying AlphaGo to create algorithms and programs to assist humans in fields from medical care to scientific research and robotics.