Fribourg, Switzerland  - Calls by Al-Qaeda extremists to attack Switzerland and Austria during Euro 2008 are being taken seriously by authorities, a senior Swiss security official said.

Messages on "sos minbar" and "As-Sahab", two internet sites favoured by the terrorist group and calling for attacks on the two host nations, had multiplied in recent weeks, the official was quoted as saying by La Liberte daily published Thursday.

"We are taking these threats seriously," he said.

"We are on alert and we are following these jihad forums very closely. It is through these that (Osama) Bin Laden's agents awaken dormant cells. The situation is serious even if it is frustrated people hiding behind these sites."

According to the official, one site said: "Let's transform the two most secure countries in Europe into hell, like the hell in Iraq and Afghanistan."

Another message said: "The hour has come for fighters of the faith. They must make themselves heard."

Euro 2008 kicks off on June 7 in the Swiss city of Basle. Millions of football fans are expected to travel to the games shared between eight venues in Switzerland and four in Austria for what is one of the world's biggest international sporting events. (dpa)