Germany's [Images] Federal Crime Office (BKA) believes at least 21 matches during the football World Cup will be at high risk of terrorist attacks, a magazine reported on Wednesday.

The danger of attacks by Islamic extremists was especially high, weekly magazine Stern reported in its latest edition, citing a confidential report by the BKA.

The report said the tournament opener in Munich between Germany and Costa Rica on June 9, and the final on July 9 were at particular risk, as were matches involving the United States, due to its leading role in the global "war on terror".

Ties involving England [Images], Spain, Poland and Australia presented further potential targets, because their governments backed the U.S.-led invasion of Iraq in 2003, the report added.

A spokesman for the BKA said it was no secret the Federal Crime Office compiled situation reports on the World Cup.

"But we will make no statement on this, as we do not comment on internal reports and documents on principle," he said.

German authorities have been at pains to stress the country has no intelligence of any specific threats to the football tournament, which will feature teams from 32 countries.

Germany has not suffered a major militant attack in recent years, although a cell including members of the group behind the September 11 attacks on the United States was based in Hamburg.