GAZA CITY, Gaza Strip - The conflict in the Gaza Strip between Israeli forces and Hamas militants saw its deadliest day since it began nearly two weeks ago with more than 60 Palestinians and 13 Israeli soldiers killed Sunday. The deaths amid unconfirmed claims by Hamas late Sunday that they'd captured an Israeli soldier.

Palestinian President Mahmoud Abbas declared three days of mourning and accused Israel of carrying out a massacre in Shejaia in the eastern suburbs of the city of Gaza, while Israel said it was targeting militants from Gaza's dominant Hamas group whom it alleged had fired rockets from Shejaia and built tunnels and command centers there.

Locals had allegedly been warned to leave by Israel two days ago, but in the heavily over-populated Gaza Strip, there is often nowhere to go. Residents fled Sunday's fighting along streets strewn with bodies and rubble, many of them taking shelter in Gaza's Shifa hospital.

The 13 Israeli soldiers killed was the Israeli military's highest one-day death toll since a 2006 war against Hezbollah guerrillas in Lebanon. According to army sources, seven of those killed were in an armored personnel carrier hit by anti-tank fire.

Naser Tattar, the Shifa hospital director, confirmed 17 children, 14 women and four elderly were among the 62 Palestinians killed, with a further 400 people wounded in the Israeli assault.

Gaza's Health Ministry officials said the death toll in the Gaza Strip had reached 435 Palestinians, many of them civilians, with about 2,600 wounded since Israeli air and naval bombardments began on July 8, followed by a ground push on Thursday.

On the Israeli side, 18 soldiers and two civilians have been killed since the offensive was launched in response to mounting cross-border rocket attacks by militants. Despite the ground offensive and air strikes, Hamas militants kept up their rocket fire on Israel on Sunday.

The Israeli military said it was continuing to expand its ground offensive on Sunday, with a focus on destroying missile stockpiles and a vast tunnel system Hamas has built along the frontier with Israel.

Israeli Prime Minister Benjamin Netanyahu accuses Hamas of using non-combatants as human shields, and has repeatedly insisted the army was concentrating on military targets amid growing civilian casualties.