TOKYO, Japan (AFP) — Naoya Inoue of Japan said he had not yet reached his "goal" after becoming the first undisputed bantamweight world champion in half a century Tuesday with a knockout win over England's Paul Butler.

The unbeaten Inoue, nicknamed "Monster," had to work hard to grind down a defensive Butler in Tokyo but finally dropped him to the canvas with a flurry of shots in the 11th round.

The win saw Inoue achieve his long-held ambition of becoming the first undisputed bantamweight world champion since Panama's Enrique Pinder in 1972, adding Butler's World Boxing Organization title to his own World Boxing Confederation, World Boxing Association and International Boxing Federation belts.

But he warned that his rampage through the boxing world was not finished yet and that he will review his options before making his next move.

"This is not the goal," said the 29-year-old Inoue, who is widely expected to make the step up to the super-bantamweight division.

"Tonight, was something great to see with the fans from where I was standing in the ring, but this is just a point along the way."

"Do I feel satisfied? I am satisfied but I want to turn my mind to what comes next."

Inoue becomes only the ninth undisputed world champion since the four-belt era began in 2004, and the first in the bantamweight division.

He took his record to 24-0, with 21 knock-outs, after finally managing to put away underdog Butler, who was fighting away from home for the first time.

Inoue started the bout in characteristically ferocious fashion, landing several big shots in the opening round.

He continued to punish Butler in the early stages but the Englishman held firm and refused to emerge from his defensive shell.

Inoue grew frustrated by Butler's approach and praised his opponent afterwards for his "thorough game plan."

"He tried to get through the first half and then make it a contest from the middle of the fight onwards," Inoue said.

"I expected that but he had a good plan to stop me. I had to change my own style and try to coax him out."

Inoue finally broke down Butler's resistance with a juddering body shot in the 11th, then made sure he went down with a volley of punches.

Butler paid tribute to Inoue as a "very, very good fighter" and acknowledged that he "fell just short."