Palestinian medics said 10 civilians, including five children, were among those killed. Six other fatalities were believed to be militants from Hamas and the Islamic Jihad group.

Israel launched an air offensive in response to increasing rocket attacks by Palestinian militants, conducting repeated strikes that killed at least 16 Palestinians on Tuesday in the Gaza Strip.

Palestinian medics said 10 civilians, including five children, were among those killed. Six other fatalities were believed to be militants from Hamas and the Islamic Jihad group.

Israel’s Cabinet, meanwhile, authorised the Army to call up 40,000 reservists should it decide to expand the operation, possibly into a ground offensive. So far, only thousands of reserve soldiers have received orders.

Five Palestinian militants were killed at dusk by Israeli soldiers during an attempt to launch an attack by sea on Israeli territory at the Zikim military base just north of the Gaza Strip. Israel Radio reported heavy exchanges of fire.

Following rising tensions and violence in the region, Israel said it had conducted about 200 airstrikes on the coastal enclave since early Tuesday.

Hundreds of rockets and mortar shells have landed in southern Israel in the last two days, causing some damage to property and minor injuries. It is the first time Hamas has claimed responsibility for rocket fire into Israel since November 2012, when Israel launched its last major offensive against the Gaza Strip.

Israel Prime Minister Benjamin Netanyahu blamed Hamas for the escalation.

“We will not tolerate the firing of rockets into our cities and towns, and therefore I have ordered a significant expansion of the IDF’s action against the terrorists from Hamas and against the terrorists from other terrorist organizations in the Gaza Strip,” he said.

Earlier, Defence Minister Moshe Yaalon said the campaign will not end within a few days. He said the offensive against Hamas would continue until the group agreed to a ceasefire on Israel’s terms.

According to Palestinian medics, at least 100 people were injured in the airstrikes and several homes belonging to militants were destroyed.

Palestinian President Mahmoud Abbas called for Israel to “stop the escalation and the attacks on Gaza without delay,” according to the official Wafa news agency.

Israeli military spokesman Peter Lerner said the offensive could grow into a “ground mission if required”. The escalation in violence came on the heels of growing tension in the West Bank, especially since the abduction and slaying of three Israeli teens, which Israel blamed on Hamas, though the group denies responsibility.

A Palestinian teen was then burned alive last week, further inflaming tension. Israeli police arrested six Jewish suspects, believed to be radicals seeking retribution.

Hundreds of Hamas activists were arrested and the movement’s institutions were shut down in recent weeks in the West Bank.

Hamas claimed responsibility on Monday for the rocket fire, which in the last two years has been attributed to smaller groups, after seven of its fighters died in a tunnel collapse, which the group blamed on an Israeli airstrike.

“The Palestinians today are carrying out a struggle against a Zionist government which should be referred to the International Criminal Court for the crimes it is committing against the Palestinians,” said Ali Barakeh, Hamas spokesman in Lebanon.

Mr. Barakeh urged Arab countries to side with the Palestinians amid growing schisms among countries in the region as a result of the Arab Spring.

The Arab League is likely to push for a meeting of the UN Security Council on the latest Israeli offensive, reports said. Izat al-Resheq, a senior Hamas leader, wrote on his Facebook page that the militant movement was “ready to fight to the end” if Israel launched “a new war” on the Gaza Strip.

Analysts said Hamas is weaker now — financially, militarily and diplomatically — as it has lost the backing of Iran, Syria and supporters in several oil-rich Gulf countries because of its political positions during the Arab Spring.

Sirens have been going off regularly in southern Israel since last week, warning residents to seek shelter from rockets.

Schools, kindergartens and summer camps were shuttered within 40 kilometres of the Gaza Strip. Police banned any gatherings of more than 300 people in the region.

“This is a very difficult situation that requires real mental strength,” said Hanna Tal, a social worker in the Shaar HaNegev regional council, bordering Gaza. “For the past week and a half, there are constantly mortars and rockets, and this really stresses the people out.” Israel said rockets fell in Jerusalem, causing damage to property, while the military said it intercepted a rocket heading toward Tel Aviv. Several other rockets landed along the coast.

The United States condemned the rocket fire into Israel and the striking of civilian targets. White House spokesman Josh Earnest stressed U.S. concern about the safety of civilians on both sides.

“It is not in the interest of either side for this violence to continue and even to escalate,” he said, “so we are hopeful that even as Israel exercises their right to self-defence that they’ll leave open a channel for diplomacy to prevail and for a ceasefire, or at least a de-escalation in the violence, to commence.” U.N. Secretary-General Ban Ki-moon condemned rocket attacks by Palestinian militants, saying the “indiscriminate attacks on civilian areas must stop”. He called for all actors to “exercise maximum restraint” and avoid “further civilian casualties and overall destabilisation”.

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