Palestinian rocket attacks on Israel and IDF artillery strikes near populated areas in northern Gaza constitute serious violations of the laws of war, a leading human rights group said in a report released Sunday. The Human Rights Watch report spanned the period between September 2005 and May 2007, during which four Israelis and at least 61 Palestinians were killed and some 370 people - most of them Palestinians - were wounded. Palestinian armed groups and Israeli soldiers both have shown "insufficient regard for civilian life," the rights group said in its 146-page report. Palestinian claims that the attacks on civilians are reprisals for Israeli actions are not justification because "reprisals against civilians are always illegal," the report said. Casualties also jumped after Israel reportedly narrowed the buffer zone it places between artillery targets and civilian areas and stepped up shelling in April 2006 following the Hamas group's political takeover of the Palestinian Authority, the report said. "Tit-for-tat abuses can't be justified by arguing that the other side violated the law first: The laws of war are meant to protect civilians from harm, whatever the reason," said Joe Stork, deputy director of Human Rights Watch's Middle East division. "Otherwise, the cycle of violence spirals out of control, as happened in Gaza and Israel," he says. The IDF objected to the report's equation of Palestinian rocket attacks deliberately aimed at causing Israeli civilian casualties and IDF military fire employed in defense against these very attacks and directed at legitimate military targets. Chief Palestinian negotiator Saeb Erekat said the two sides were not equally at fault. "I cannot accept the equation between Israel as an occupier and Palestinians as people under occupation," Erekat said. "We want to stopâ€¦ rockets but we cannot equate between what Israel is doing in Gaza and the West Bank with what Palestinians do." From September 2005 until May, Palestinian gunmen fired almost 2,700 locally made rockets toward Israel, killing four Israeli civilians and wounding 75, according to Human Rights Watch. Palestinian rockets also caused at least 23 Palestinian casualties, including two deaths, when they fell short of the border, the group said. Hamas spokesman Sami Abu Zuhri rejected the report's comparison to the use of crude rockets with the warplanes, attack helicopters and high-tech weapons at Israel's disposal. "It makes equivalence between an occupation and an occupied people," he said. "We are pushed to defend Palestinian civilians with the means that we have." Between September 2005 and November 2006, the IDF fired more than 14,600 artillery shells into Gaza. Shells fired close to populated areas killed 59 people and wounded 270, most, if not all civilians, the report said. Israel suspended artillery attacks into Gaza after an attack killed 23 Palestinian civilians last November 8, but says it still considers artillery fire a legitimate response to Palestinian rocket attacks, subject to the limitations of international law, the report said.