Muslim hackers have launched a massive cyberattack, defacing more than 300 Israeli websites since the Jewish state began pounding Hamas targets in the Gaza Strip, a computer forensics expert said this week.

And operators of pro-Israeli websites operating in the United States should expect similar assaults to come their way, said Gary Warner, director of research in computer forensics at the University of Alabama at Birmingham.

"In the current situation, the hackers supporting Gaza clearly believe Israel and the U.S. are culpable," he wrote on his blog. "That means American webmasters may wish to be especially vigilant right now."

Warner called the spate of defacements a "Propaganda War," in which the Muslim hackers replace legitimate content with anti-Israeli and anti-United States messages. 

The last time a similar campaign of this magnitude was unleashed was in 2006, when thousands of Danish and American sites were either hacked or hit with distributed denial-of-service attacks. Muslim extremists were angered over satirical Danish caricatures of the prophet Muhammad that appeared in a number of European newspapers.

Warner said website operators must secure common entryways for hackers, including vulnerable programming language, forum or blog software, image programs and utilities, such as website statistic applications. In addition, they must prevent the theft of FTP credentials, which are used to access sites and load content.

John Kindervag, a senior analyst with Forrester Research, said most websites are not built with security in mind. As a result, site owners should conduct a vulnerability assessment and, if they need immediate action, install a web application firewall, which can detect anomalous behavior.

"If you think about a defacement, you have to get to the core image files that make your web application work, and theoretically, you shouldn't be able to get to those from an external interface," he told SCMagazineUS.com on Wednesday.

Israeli leaders on Wednesday rejected calls for an immediate cease-fire and continue to pound Hamas targets in the Gaza Strip for the fifth straight day. Some 390 people have been killed, mostly members of Hamas security forces. 

The assault comes in response to Hamas rocket strikes into Israel, which Israeli leaders said are landing closer to densely populated cities than ever before.