Computers that were used in an Internet assault against several leading Israeli business websites on Monday are located in Israel - although the hacker apparently is not, according to Gil Shwed, CEO of Internet security company Check Point.

Among the websites targeted were those of the Tel Aviv Stock Exchange, El Al Israel Airlines and Israeli banks, and on Tuesday it was announced that the Israel Police website was also hit.

The method used by the hacker or hackers that overwhelmed the Israeli websites this week involves the use of malicious software such as a computer virus that enables a computer to be taken over without its owner's knowledge. It is then used, through a setup known as a botnet, to disseminate junk messages and computer viruses in an effort to knock websites or computer networks out of service. The technique usually involves bombarding the target website or network with a large number of external communications requests.

Among the targets of Monday's attack were marketing websites of Bank Beinleumi (First International Bank of Israel ) and two of its subsidiaries, Otzar Hahayal and Massad, but not its online banking sites.

The CEO of Maglan Information Defense Technologies, Shai Blitzblau, confirmed that the recent Internet assault, which forced the targeted websites to shut down for a time, made use of a number of Israeli computers, but he said most of the attacks this week came directly from abroad.

He suggested that the computer hacker chose to make use of computers in Israel - apparently without their owners' knowledge - because of the damage that would be caused by the disclosure of databases of passwords and as a tactical measure. Once an assault is detected, he explained, firms can more easily block contact from abroad than activity originating from computers in Israel.

This week's incident follows an attack earlier this month when a hacker calling himself 0xOmar, reportedly from Saudi Arabia, disclosed details of thousands of Israeli credit cards on the Internet.

Israeli hackers now claim to have retaliated against this week's attack on the Israel Police website by crippling the Saudi Stock Exchange Internet site yesterday, according to a statement on the Pastebin website, which has served as a forum over the past few days for claims by both sides in the cyber war.

Israeli hackers also claim to have attacked the website of the Saudi government agency that regulates stock trading as well as the Abu Dhabi Stock Exchange website. Israeli and pro-Israeli hackers also claim to have disclosed the personal details of citizens of Saudi Arabia, the United Arab Emirates and other Arab countries.

"If the attacks from Saudi Arabia continue, we will step up our activity and neutralize these sites for a longer period of weeks and months," Israeli hackers warned. Another group calling itself "Gilad Shalit" after the Israeli soldier recently released from Hamas captivity, claims to have attacked the electricity company of Afghanistan.

The potential scope of the use of computers of unsuspecting owners is huge. In November, for example, the FBI in the United States uncovered a botnet scheme that made use of 4 million computers.

At a news conference yesterday, Check Point's Shwed called for more coordination among authorities around the world to address the threat. The attack against Israeli websites this week is similar to assaults carried out on July 4, 2009, against targets in the United States and South Korea. Those attacks were thought to have come from North Korea.