Turkish Internet servers are suffering a powerful cyber attack, slowing banking services and fanning fears that it could be a politically motivated attack from abroad.
Nic.tr, a non-governmental organisation that administers addresses for websites using the "tr" domain, said Thursday that the attack appeared to be from "organised sources" outside Turkey.
The domain is used by websites belonging to Turkish ministries, commercial enterprises and banks.
Transport and Communications Minister Binali Yildirim called it a "serious" attack and asked an Ankara-based university to beef up security measures, saying they were "insufficent".
Banking sources said the attack has seriously disrupted transactions.
Experts have been unable to identify the source of the attack.
Some newspapers said it could have originated in Russia, citing fraught bilateral relations since the downing by Turkey of a Russian fighter jet last month on the Syrian border.
According to local media, the Anonymous hacking group posted a message saying it would continue to attack Turkey for "supporting the Islamic State by buying their oil and tending to their injured fighters."