Israeli jets have bombed militants in the Gaza Strip after the country's army said 25 mortar shells were fired into its territory - the biggest such attack for years.

"Most of the launches were intercepted by the IDF's (Israel Defence Forces) Iron Dome aerial defence system," said an Israeli military spokesman.

One shell exploded near a pre-school before the children had started for the day.

Sirens sounded at villages and towns near the border as the shells came in, but there were no reports of injuries.

The mortar attack is the largest from Gaza since the 2014 war, according to Israel's Shin Bet security service.

"Israel takes a grave view of the attacks on it and its communities by Hamas and Islamic Jihad from the Gaza Strip," said Prime Minister Benjamin Netanyahu.

"The IDF will respond to these attacks powerfully,"

Israel's military confirmed that explosions and smoke near the town of Deir al-Balah were related to its actions, and officials said the site was used for training militants.

Gaza security sources said at least one Hamas base and four Islamic Jihad bases had been hit, but a Hamas spokesman said only "resistance" bases were targeted.

Hamas - the group that runs Gaza - said that "what the resistance carried out this morning comes within the framework of the natural right to defend our people".

Palestinian deaths

The incident comes after a number of Palestinians were killed over the last two days - including three from Islamic Jihad.

On Monday, a tank fired at a Hamas position in Gaza, with the Palestinian Health Authority saying one man had died and another was injured.

Israel said it had retaliated after troops came under fire while trying to stop two armed Palestinians getting across the border with the "intention of carrying out an attack".

A day earlier, three Islamic Jihad militants were killed when Israeli tanks shelled one of its observation points - with the group promising to hit back.

Israel said it had attacked after an explosive device was placed on the border fence.

It all comes just two weeks after 61 Palestinians were killed during violent protests at the border on the same day the US went ahead with its controversial decision to move its embassy from Tel Aviv to Jerusalem.

Demonstrations along the border have continued at a lower level since then, and at least 116 Palestinians are said to have died since the unrest began on 30 March.

A boat carrying students and sick and injured people also left Gaza City's port on Tuesday in the latest action.

It is aiming to symbolically break Israel's sea blockade, which limits boats to six nautical miles from shore.