The websites of Israeli national carrier El Al and the Tel Aviv Stock Exchange were both offline on Monday morning hours after they were reportedly threatened by a hacker claiming to be Saudi.
Both sites had a temporary message posted which said they had been taken down for "maintenance".
A spokeswoman for the stock exchange confirmed that the site had come under attack but said only the website had been affected, not the exchange's trading systems.
"There is someone that has been attacking the Tel Aviv Stock Exchange (TASE) website since this morning," Idit Yaaron told AFP, describing what appeared to be a distributed denial of service attack (DDOS).
"The trading systems are okay, there is no problem (with them)," she said.
Earlier on Monday, a hacker who calls himself 0xOmar warned he was going to attack both websites, Israeli media reported.
The attack was the latest in a series of incidents over the past two weeks in which details of tens of thousands of Israeli-owned credit cards have been posted online and Israeli websites defaced by hackers claiming to be from Saudi Arabia or Gaza.
The cyber attacks were hailed by the Islamist Hamas movement which rules Gaza, with spokesman Sami Abu Zuhri saying it was a sign of the "creativity" of Arab youngsters.
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"The Arab hackers are resisting Israel in cyberspace and are a sign of the Arab youth's creativity in inventing new forms of Arab and Islamic resistance against the Israeli occupation," he told AFP.
"Hamas praises the Arab hackers and calls on the Arab youth to play their role in cyberspace in the face of Israeli crimes," Abu Zuhri said, urging the hackers to "ignore" threats by Israel to strike with force against anyone attacking the Jewish state.
0xOmar first struck on January 3, when he claimed to have posted details of 400,000 Israeli-owned credit cards online.
Three days later, he said he had published another 11,000 card details, but the links turned out to contain malware that infected anyone who downloaded the information.
Israel's main credit card companies said that about 20,000 valid cards had been affected.
In response, an Israeli hacker going by the name "0xOmer" published details of more than 200 Saudi-owned cards, sparking a spate of tit-for-tat attacks.
Last week, Gaza-based activists hacked the website of Israel's deputy foreign minister Danny Ayalon and on Friday, they hacked the website of Israel Fire and Rescue Services, posting a message saying "Death to Israel".
"This is the beginning of cyber war against Israel, you are not safe any more," 0xOmar wrote in a posting on Pastebin.com last week, urging hackers from across the Arab and Muslim world to target Israel.
"We'll hack Israeli servers for different purposes like leaking Israeli data, sensitive and hidden information extraction and defacing websites," he wrote.