Israel strikes Gaza after heaviest mortar barrage in years
Israel has attacked militant sites in Gaza after coming under the heaviest barrage of mortar and rocket fire from the Palestinian enclave in years.
The Israeli army said air strikes hit 35 targets, including a cross-border tunnel, belonging to Hamas and Islamic Jihad - Gaza's main militant groups.
In Israel, an empty kindergarten was hit when militants fired more than 30 mortars earlier in the day.
The flare-up follows weeks of deadly violence on the Gaza-Israel border.
More than 100 Palestinians have been killed by Israeli snipers amid protests which saw thousands of Palestinians mass on the border in support of the declared right of Palestinian refugees to return to their ancestral homes in what is now Israel.
Israel said its snipers had opened fire on attackers or people attempting to infiltrate under cover of protests, but UN and human right officials have accused Israel of using disproportionate force.
Israeli fighter jets struck seven sites across the Gaza Strip, hitting what the Israeli military said were "six military compounds, munition storage warehouses, naval targets, and terror headquarters".
Israeli Defence Minister Avigdor Lieberman said: "Hamas and Islamic Jihad have already paid a heavy price and the bill has just been presented to them."
Gaza's Hamas-run Ministry of Education said a school was hit by shrapnel from Tuesday's air strikes, though there were no initial reports of Palestinian casualties.
The Israeli military said its air force also struck "an offensive Hamas terror tunnel" near the Kerem Shalom crossing, where controlled amounts of food, fuel and goods are transferred into Gaza.
It said the tunnel stretched for 900m (3,000ft) under Israeli territory. It is the latest in a series of cross-border tunnels which Israel has destroyed or disabled since the end of the 2014 Israel-Gaza war.
During that conflict, Israel destroyed more than 30 tunnels which it said were meant for attacks.
The Kerem Shalom crossing is a lifeline for Gaza, which has been under an Israeli, then Egyptian, blockade beginning in 2006 when Hamas militants attacked the crossing and kidnapped an Israeli soldier.
The blockade was tightened after Hamas, an Islamist group which won Palestinian elections in 2006, forcibly ousted its secular Fatah rivals from Gaza in June 2007.
Sirens warning of incoming fire rang out in Israeli communities around the Gaza Strip on and off on Monday.
The Israeli military said the biggest volley of mortar shells was fired at several sites in Israel in the early hours, with most intercepted by the Iron Dome anti-missile defence system.
Some mortars landed, included one in the grounds of a kindergarten in the Eshkol region, which borders Gaza, shortly before pupils were due to arrive.
More shells were launched in subsequent attacks, the Israel Defense Forces (IDF) said. Three Israelis were wounded, media reports said.
The UN Middle East peace envoy, Nickolay Mladenov, condemned the attacks on Israel, saying he was "deeply concerned by the indiscriminate firing of rockets by Palestinian militants from Gaza.
"Such attacks are unacceptable and undermine the serious efforts by the international community to improve the situation in Gaza".
Hours earlier, machine-gun fire from Gaza hit houses and vehicles in the Israeli border town of Sderot, though without causing injuries, the IDF said.
The upsurge in violence came after Israeli tank fire killed four militants in Gaza in two separate incidents at the start of the week.
A member of Hamas was killed on Monday after Israeli soldiers caught a group attempting to breach the border and carry out an attack, while on Sunday three members of Islamic Jihad were killed after placing an explosive device on the border fence, the IDF said.