A group of West Ham United fans were attacked in Frankfurt early Wednesday on the eve of the Europa League semi-final second leg against Eintracht Frankfurt, German police confirmed.

A police spokesman said masked groups in two minibuses stopped outside a pub in Frankfurt's Bockenheim district where travelling West Ham supporters were drinking.

"A group of about 15-20 people got out, went in and are said to have physically attacked the guests," police said in a statement.

"A 34-year-old man from the UK was taken to hospital with minor injuries."

The attackers drove away from the scene once police were alerted. A search was unsuccessful.

Witnesses said the groups of attackers were wearing clothing linking them to Eintracht Frankfurt.

On Tuesday, two British males were also set upon in Frankfurt's Sachsenhausen district by six men identified as Eintracht Frankfurt fans, who were later arrested.

Tensions are running high and a big police presence is expected for Thursday's semi-final in Frankfurt after the German club won the first leg 2-1 at West Ham last Thursday.

There was already trouble during the first leg when German reporters Tim Brockmeier and Philipp Hofmeister were attacked by West Ham fans at the London Stadium.

West Ham are investigating and have reportedly given the names of two supporters to police.