Israel has begun reducing the amount of electricity it sells to Gaza as part of sanctions against continued rocket fire, Israeli officials said Friday. The move prompted a warning from the United States not to "worsen the humanitarian situation" of civilians in Gaza, and was followed by the firing of yet more rockets at Israel by militants there.

Israel began reducing the electricity flow into the Gaza Strip by less than 1 percent late Thursday night. By Friday afternoon, 21 rockets had been launched against Israel, an army spokeswoman said, with several landing in and around the Israeli border town of Sderot and in open areas south of Ashkelon, a larger Israeli coastal city north of the strip. The rockets caused slight property damage, but no one was hurt.

Israeli officials said the electricity had been cut by about one megawatt out of the roughly 124 megawatts that Israel provides to Gaza, and that an additional megawatt could be cut each week depending on the security situation and the needs of the Gazan population. Israel said it would continue to provide the necessary minimum to prevent harm to the safety or health of the residents.

"It will take time before it really has an effect," said Shlomo Dror, a spokesman for the Defense Ministry.

But Israeli and Palestinian human rights groups contend that the cut in electricity, along with Israeli-imposed restrictions on fuel supplies, violates international law and deprives Gaza's residents of the energy needed to run vital services. The Israeli Supreme Court rejected a petition by 10 human rights groups more than a week ago to stop Israel from further reducing supplies of fuel and electricity to Gaza.

Israel has restricted the amount of fuel allowed into Gaza since last autumn after declaring the area, controlled by the militant Islamist group Hamas, to be "hostile territory." The importation of industrial diesel needed to run the only power plant in Gaza has been limited, preventing the station from running at full capacity and leading to electricity rationing and rolling blackouts in Gaza.

The power plant, which can generate up to 80 megawatts of electricity, is now producing roughly 55 megawatts, human rights organizations say. Gaza receives 17 megawatts from Egypt.

Human Rights Watch, based in New York, said Thursday that the fuel and expected electricity cuts "amount to collective punishment."

Tom Casey, a spokesman for the State Department, said, "We understand Israel's right to defend itself, but we do not think that action should be taken that would infringe upon or worsen the humanitarian situation for the civilian population in Gaza."

Deflecting the criticism, the Israeli infrastructure minister, Benjamin Ben-Eliezer, told Israel Radio that "no country in the world would agree to provide electricity to people who shoot at the electricity plant which supplies it to them." He was referring to rockets that have been fired from Gaza at the power station in Ashkelon.

Dror of the Defense Ministry said Friday that the Israeli "process of disengagement" from Gaza was proceeding, and that "instead of investing in firing rockets," the Palestinians of Gaza could look for other sources of electricity, either by buying more from Egypt or by building another local power plant that could receive fuel from Egypt.

Some Israeli officials, including Deputy Defense Minister Matan Vilnai, suggested that the electricity cut was not punitive, but rather part of the effort to reduce Gaza's dependence on Israel in different fields.

Separately, Yehezkel Dror, an Israeli political science professor who sat on the investigative commission that criticized Israeli political and military leaders for their handling of the 2006 Lebanon war, said Friday that he was not satisfied with the way the commission's final report had been presented to the public. He criticized the news media for what he said had been a "disproportionate focus" on the political fallout from the report at the expense of the fundamental systemic problems raised in the report.

The commission's preliminary report, made public in April, contended that the prime minister, Ehud Olmert, as well as the wartime defense minister and army chief of staff, were personally responsible for severe failings in the conduct of the war. The commission's final report, published Jan. 30, did not assign personal responsibility by name, in part because of conditions imposed by the Supreme Court.

As a result, the final report was generally seen as being less critical of Olmert, who has remained in office.

The commission was led by a retired judge, Eliyahu Winograd. Dror is the only one of the five-member panel to have spoken extensively to the news media and in public forums since. He emphasized that he was speaking in a personal capacity, and no longer as a representative of the commission.

Meeting with a group of reporters here on Friday morning, Dror said, "I doubt that most politicians or television commentators who comment on the report have read it." The censored version of the report is more than 600 pages long.

A prominent Israeli author who lost a son in the 2006 war, David Grossman, described the report as "circuitous, overly cautious," in an article on Friday in the newspaper Yediot Aharonot. Grossman described the country's leadership as "mediocre" and "feeble" in the wake of the war, and wrote that "Israeli society cannot start to recover as long as Ehud Olmert is its leader and guide." Grossman called for the formation of an apolitical bipartisan group that "could sweep in its wake the multitudes of people who are thoroughly fed up with what is going on."