JERUSALEM — Israel faced an escalating cyberwar on Monday as unknown attackers disrupted access to the symbolically strategic Web sites of the Tel Aviv Stock Exchange and El Al, the national airline.

A hacker identifying himself as oxOmar, already notorious for posting the details of more than 20,000 Israeli credit cards, sent an overnight warning to Israel’s Ynet news outlet that a group of pro-Palestinian cyberattackers called Nightmare planned to bring down the sites in the morning.

The attackers did not break into the sites’ operating systems, but used a far simpler tactic: creating an overload of access attempts. Neither the Israeli economy nor flights in and out of the country were endangered, and the sites appeared to be recovering within hours, but the assault left many Israelis feeling vulnerable.

Yoni Shemesh, who is responsible for the Tel Aviv Stock Exchange Web site, said that his team had already begun preparing for an attack a few days ago and went on high alert after the Ynet report.

“We are putting up blocks to the hackers,” he told Israel Radio at noon, about three hours after the attack started. “It is a real cyberwar.”

El Al said it took down its site as a cautionary measure as soon as unusual activity was noticed about 10 a.m.

The Web site usually sees about 50 simultaneous access requests at any given moment in the morning hour, according to a spokeswoman for the airline, speaking anonymously per policy. “As soon as we saw that the number had risen to about 1,000, we closed it down,” she said.

A spokesman for Hamas, the Islamic militant group that controls Gaza, praised the hackers for opening up a new resistance front against Israel.

Israel is widely considered a technological powerhouse and a hub of high-tech industry. But Avi Weissman, the chief executive of See Security InfoSec and Cyber Warfare College and chairman of the Israeli Forum for Information Security, a nonprofit organization, told the radio that in the cyberrealm, Israel may be a power “in terms of attack, but in terms of defense, we are a very small and pretty neglected country.

Professor Yitzhak Ben Yisrael, a military scientist who has advised the government on cybersecurity, said that the country had been working to protect its most vital systems in the security establishment and in the civilian sphere, including electricity, water and trains, for more than 10 years.

The hacker oxOmar described himself as a 19-year-old from Riyadh, Saudi Arabia, when he posted the details of the thousands of Israeli credit cards, turning himself into a household name here. A few days later, a hacker presenting himself as an Israeli and going by the name oxOmer — a twist of the Saudi handle — countered by posting details of what he said were more than 200 Saudi credit cards. But he did not expose the security codes, saying his aim was only to “alert.”