* Banks report sporadic disruption
* Wave of attacks has gone on for two weeks
* Network specialist says barrage is intensifying
By Can Sezer and Ebru Tuncay
ISTANBUL, Dec 25 (Reuters) - Turkish banks reported sporadic disruption to credit card transactions on Friday as hackers stepped up a two-week barrage of cyberattacks, believed to be the worst the country has seen.
Local media have suggested that the bombardment of public and financial websites could be coming from Russia, after a sharp worsening of tensions between Moscow and Ankara, or staged by hacking group Anonymous. But no clear evidence has emerged, and authorities have avoided pointing the finger.
Officials at several Turkish banks including Isbank , Garanti and state lender Ziraat Bank confirmed the attacks, saying they had caused intermittent disruption. Bank shares were unmoved by the news.
"The attacks are serious," said Onur Oz, a spokesman for Internet provider Turk Telekom. "But the target is not Turk Telekom. Instead, banks and public institutions are under heavy attack," he said.
"A majority of Turkish institutions use Turk Telekom as the service provider, therefore we are the ones doing the defence against these attacks."
A government cyber security unit was taking precautions and the incident was under control, Transport Minister Binali Yildirim said earlier this week. He did not comment specifically on the targeting of banks, and his ministry was not available for comment on Friday.
"These attacks began two weeks ago but have intensified over the past two days," said Burak Atakani, a network specialist from Istanbul Technical University.
He said that the service provider used by some universities, police academies and the research department of the military had also come under attack.
Nic.tr, a non-government body that administers addresses for websites using Turkey's ".tr" domain, including ministries, the military, banks and many commercial sites, said earlier this week that the attacks originated from "organised sources" outside Turkey.
Turkey incensed Russia last month by shooting down a Russian military plane near the Syrian border, and Moscow has retaliated with economic sanctions. In the past, countries such as Estonia and Georgia have suffered cyberattacks emanating from Russia after incurring Moscow's anger.
Reuters sought comment from the Russian foreign ministry, but none was immediately available.
Separately, the Anonymous hacking group posted a video over the weekend on YouTube, which has since been removed, saying cyberattacks would continue if Turkey "doesn't stop supporting" Islamic State.
Some of Ankara's critics, including Russian President Vladimir Putin, have accused Turkey of supporting and aiding Islamic State militants, a charge the government denies. (Additional reporting by Ceyda Caglayan and Melih Aslan, and Ludmila Danilova in Moscow; Writing by Humeyra Pamuk; Editing by David Dolan and Mark Trevelyan)