The army said it was "currently striking terror targets in the Gaza Strip", without giving further details, following an assault on the territory earlier in the day.

According to Palestinian security sources and witnesses, fighter jets struck Hamas targets without causing any injuries.

The Israeli bombardment came as rockets and mortars were lobbed into southern Israel from the blockaded Palestinian enclave.

Sirens warning Israeli residents to take shelter rang out in all areas bordering Gaza, the army wrote on Twitter.

The escalation followed further protests on the Gaza border on Friday, in which two Palestinians were killed and more than 220 injured according to the health ministry in the Hamas-run Strip.

A 15-year-old Palestinian was shot dead during the demonstrations and a 20-year-old died Saturday of gunshot wounds sustained during the protest, the health ministry said.

The Israeli military said one of its soldiers was injured by a grenade.

- Israel hits Gaza tunnels -

Early on Saturday the Israeli military said it conducted strikes against Hamas "in response to the terror acts instigated during the violent riots that took place along the security fence" on Friday.

It also cited "continuous arson attacks damaging Israeli territory on a daily basis with the launching of arson balloons from the Gaza Strip into Israeli territory".

In the morning raids Israeli fighter jets struck two "Hamas terror tunnels", one in southern Gaza and another in the north, as well as other infrastructure across the coastal territory.

It said targets included "complexes used to prepare arson terror attacks and a Hamas terror training facility".

During the morning strikes, militants in Gaza fired a total of 31 mortar rounds at Israel, the military said, adding that six were intercepted by its Iron Dome air defence system.

Israeli media reports said no one was injured in those attacks.

Hamas spokesman Fawzi Barhoum said the group was responsible for the mortar fire on Israel and that they were carried out "in response to the Israeli air strikes".

"The protection and the defence of our people is a national duty and a strategic choice," Barhoum said.

- Grenades and flaming tyres -

Palestinians in Gaza have for months been demonstrating against Israel's decade-long blockade of the territory and in support of their right to return to lands they fled or were driven from during the war surrounding the creation of Israel in 1948.

Since the protests and clashes broke out along the border on March 30, at least 141 Palestinians have been killed by Israeli fire.

The majority were involved in protests and clashes but others were seeking to breach or damage the border fence.

No Israelis have been killed.

Israel's army said grenades, Molotov cocktails, flaming tyres and stones have been hurled in the direction of its soldiers.

Israel says its use of live fire is necessary to defend its borders and stop infiltrations.

It accuses Gaza's Islamist rulers Hamas of seeking to use the protests as cover for attacks against Israel.

Hamas has run Gaza since seizing it from the Palestinian Authority amid deadly clashes in 2007.

Since then, Israel has maintained a crippling blockade on Gaza which it argues is necessary to isolate Hamas.

Israel and Hamas have fought three wars since 2008.