Leading media websites in the Czech Republic say they have come under repeated and powerful cyberattacks.

The online editions of Mlada Fronta Dnes, a major newspaper, and of the respected business daily Hospodarske Noviny were among the affected. The attacks occurred Monday morning and again in the afternoon.

Mlada Fronta Dnes' iDNES.cz says it considers the denial of service attacks the biggest it has ever faced. It says they likely originated from infected computers abroad.

Lucie Tvaruzkova, editor-in-chief of IHNED.cz, the site that belongs to Hospodarske Noviny, says "it was a coordinated attack on big media websites in the Czech Republic."

Tvaruzkova says it is not clear who is behind the attacks.