Gaza City Gaza Strip ? Israel ignored mounting international calls for a cease-fire Monday and said it won’t stop its crippling 10-day assault until “peace and tranquility” are achieved in southern Israeli towns in the line of Palestinian rocket fire.

Israeli forces seized control of high-rise buildings and attacked smuggling tunnels and several mosques in a relentless campaign against Hamas militants that took an increasing toll on civilians. The United Nations said at least 500 people have died in the Gaza fighting, about a quarter of them civilians.

Israel also suffered casualties. Late Monday, three Israeli soldiers were killed by what Israeli officials said was an errant tank round from one of its own guns.

Arab delegates met with the U.N. Security Council in New York, urging members to adopt a resolution calling for an immediate end to the attacks and a permanent cease-fire. At the same time, diplomats and European leaders traveled the region in an effort to stop Israel’s expanding ground and air offensive.

In a serious urban clash, Israeli troops and Hamas militants fought a gunbattle on the outskirts of the crowded Gaza City neighborhood of Shajaiyeh, Israeli defense officials said. Details also emerged of an unsuccessful attempt by Hamas fighters to capture Israeli soldiers hours after the ground operation began Saturday with a withering round of artillery fire.

Military spokesman Brig. Gen. Avi Benayahu told Israeli TV the assault was going according to plan with forces sweeping through Palestinian rocket launching locations near the border.

Despite Israeli claims that casualties have been heavy among militants, no injured Hamas fighters were seen Monday by an Associated Press reporter at Shifa Hospital, the Gaza Strip’s largest. Instead, the hospital was overwhelmed with civilians. Bodies were two to a morgue drawer, and the wounded were being treated in hallways because beds were full.

At least 20 Palestinian children were killed during the day Monday, said Dr. Moaiya Hassanain, a health official. Most confirmed deaths have been civilians.

Three brothers died in an attack on a town outside Gaza City, a Gaza health official said. They were carried to a cemetery in an emotional funeral. One of them, Issa Samouni, 3, was wrapped in a white cloth, showing only his pale, yellow face. A man delicately placed him in a dark grave cut into the earth.

Later, five civilians were killed when a shell fired by an Israeli ship hit their house on the Gaza shore, hospital officials said. Palestinians said Israeli attacks intensified before dawn today.

Gaza health officials reported that since the campaign began on Dec. 27 more than 550 Palestinians have been killed and 2,500 wounded, including 200 civilians. U.N. humanitarian chief John Holmes put the Palestinian toll at 500, with about 125 of them civilians.

Holmes called the Gaza strife an “increasingly alarming” humanitarian crisis, directly contradicting Israeli denials that its offensive caused the growing problem. He said Gaza is running low on clean water, power, food, medicine and other supplies since Israel began its offensive. Israeli leaders have maintained there is no humanitarian crisis, and that they have been delivering vital supplies.

Late Monday, the Israeli military said three soldiers were killed and 24 others wounded by friendly fire when a tank shell hit their position outside Gaza City. The military said a colonel who commanded an infantry brigade was among the injured.