Gaza City: Israeli shelling killed eight people in southern Gaza on Friday, medics said, just hours after a 72-hour humanitarian ceasefire took effect.

A doctor at a nearby Abu Yusef al-Najjar hospital gave no immediate details of the identities of those killed, but said they had died in a bombardment east of the southern city of Rafah, after an AFP correspondent saw heavy shelling of the area.

Health Ministry official Ashraf al-Kidra and Gaza police spokesman Ayman Batniji told new agency AP that 15 other Palestinians were wounded in the shelling east of the southern Gaza city of Rafah.

An Israeli Army spokesman in Jerusalem said the military was looking into the incident.

A 72-hour ceasefire began in Gaza on Friday. The ceasefire, announced by the U.S. and the U.N. hours earlier, took effect at 8am local time on Friday after heavy fighting that killed 17 Palestinians and five Israeli soldiers.

Israel and Hamas agreed to halt all aggressive operations and conduct only defensive missions. 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.

Shortly before 10am Israeli tanks shelled the eastern part of the town of Rafah in southern Gaza, killing at least four people and wounding 15, said Health Ministry official Ashraf al-Kidra and Gaza police spokesman Ayman Batniji.

An Israeli Army spokesman said a heavy exchange of fire had erupted in the Rafah area, without providing further details.

The Gaza war has killed more than 1,450 Palestinians, mainly civilians, and more than 60 Israelis, nearly all soldiers.

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."

The cease-fire was intended to allow Palestinians in Gaza 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.