Israel and Hamas began observing a three-day Gaza cease-fire on Friday as diplomats worked to broker a lasting truce in a 25-day-old war that has killed more than 1,450 Palestinians, mainly civilians, and more than 60 Israelis, nearly all soldiers.

The cease-fire, announced by the U.S. and the U.N. hours earlier, took effect at 8:00 a.m. (0500 GMT) Friday after heavy fighting that killed 17 Palestinians and five Israeli soldiers. The cease-fire appeared to be holding, however, with no immediate reports of violence in the hour after it took effect.

Israel and Hamas agreed to halt all aggressive operations and conduct only defensive missions to protect their people. But U.S. Secretary of State John Kerry cautioned there were "no guarantees" that the lull would bring an end to the war, now in its fourth week.

Israel launched an aerial campaign against Gaza aimed at halting Palestinian rocket fire on July 8 and later sent in ground troops to target launch sites and tunnels used by Hamas to carry out attacks inside Israel.

At least four short humanitarian cease-fires have been announced since the conflict began, but each has been broken within a few hours by renewed fighting. Friday's temporary cease-fire was the longest to be announced thus far.

Under the cease-fire, Israeli troops on the ground in Gaza can continue to destroy tunnels along the heavily guarded frontier, but only those that are behind Israeli defensive lines and lead into Israel.

Israeli Prime Minister Benjamin Netanyahu on Thursday had vowed to destroy Hamas' tunnel network "with or without a cease-fire." But military spokesman Moti Almoz told Army Radio on Friday that Israel would not be able to eliminate the tunnel threat "100 percent."

At the same time, Palestinians in Gaza will be able to receive food, medicine and humanitarian assistance, bury their dead, treat the wounded and travel to their homes. The lull can also be used to make repairs to water and power infrastructure.

Egypt issued a statement early Friday calling on the Western-backed Palestinian Authority and Israel to send negotiation teams to Cairo to discuss "all issues of concern to each party within the framework of the Egyptian initiative."

Egypt had put forth a cease-fire proposal a week after fighting began last month. Israel accepted the proposal, but Hamas, which deeply mistrusts Egypt following last summer's overthrow of an Islamist government in Cairo, rejected it.

Hamas has demanded the lifting of an Israeli and Egyptian border blockade imposed on Gaza in 2007 when the Islamic militant group seized power, as well as the release of Palestinians rounded up in the West Bank in June following the killing of three Israeli teenagers.

In recent weeks Turkey and Qatar, which have warmer ties to Hamas, have tried to help broker a cease-fire agreement, with no results.

It's not clear whether other nations will attend the Egypt talks, and aides to Kerry said Egypt will ultimately decide who will participate. A Hamas official in Qatar said Hamas and Islamic Jihad officials would be participating in the Egypt talks. Israel will not meet directly with members of either group because it considers them terrorist organizations.

Hours before the cease-fire was to take effect, 17 Palestinians were killed in Israeli strikes, including 10 from the same family, said Gaza health official Ashraf al-Kidra. He said the family members were killed in an airstrike on their home in the southern Gaza town of Khan Younis.

Israel's military said five of its soldiers were killed along the Gaza border Thursday evening by a mortar round.

More than 1,450 Palestinians, mainly civilians, have been killed since hostilities began July 8, according to Palestinian officials. Israel says 61 of its soldiers and three civilians in Israel have been killed.

Hours ahead of the cease-fire, Gaza police reported heavy Israeli tank shelling in northern and eastern Gaza, and the loud exchange of fire with militants could be heard across Gaza City. Tank shells slammed into the city itself, setting homes and shops ablaze.

Hamas fighters hit an Israeli tank with an anti-tank missile, Gaza police said, and then attacked Israeli troops who came to evacuate the tank crew. Clashes continued into the early morning hours, police said.

The Israeli military said it was looking into the matter.

Israeli police meanwhile warned residents to stay away from Israeli communities near the Gaza border during the cease-fire, saying the area remains "a war zone."

"We ask the public to heed the orders of the police and army and not to go to the Gaza Strip border area, it is a threat to your life!!!" the police said in a statement.

Police said Palestinians clashed with Israeli security forces in a number of neighborhoods in east Jerusalem, and that Israelis attacked an empty bus. Police also restricted the entry of worshippers to a key Muslim holy site in the city to prevent disturbances.