At least six Palestinians have been killed as a fresh wave of fighting erupted between Israeli forces and Palestinian militants in the Gaza Strip on Sunday.
The violence came as Israel and Hamas had appeared to be making progress towards ratcheting down months of border violence.
It was not immediately clear what set off the sudden, late-night burst of violence.
In a statement, Hamas’ armed wing, the Qassam Brigades, said Israeli undercover forces in a civilian vehicle infiltrated two miles into Gaza and fatally shot one of its commanders.
It said militants discovered the car and chased it down, prompting Israeli airstrikes that killed “a number of people.”
The clashes were still going on, it said.
The Israeli military reported an “exchange of fire” had taken place during operational activity in Gaza and said that “all IDF soldiers back in Israel”.
It did not elaborate.
In Israel, the military said it had intercepted two rockets fired from Gaza as air raid sirens continued to sound.
Prime Minister Benjamin Netanyahu, on an official visit to France, announced he was going back to Israel to deal with the crisis.
Sunday’s development shattered what appeared to be a turning point after months of bloodshed along the Israel-Gaza border, with weekly Hamas-led protests drawing thousands to the perimeter fence with Israel.
More than 170 Palestinians have been killed by Israeli fire in the border protests, in which Palestinians throw rocks, burning tyres and grenades towards Israeli troops.
Last week, Israel allowed Qatar to deliver 15 million dollars (£11.6 million) in aid to Gaza’s cash-strapped Hamas rulers. Hamas responded by lowering the intensity of Friday’s border protest.
Earlier Sunday, Netanyahu had defended his decision to allow the transfer of the Qatari money, rejecting criticism that the move had strengthened the Islamic militant group.
“Every action, without exception, has a price,” he said. “If you can’t handle the price you cannot lead. And I can handle the price.”
Israeli critics, including members of Netanyahu’s hard-line coalition, accused him of capitulating to violence and of granting relief to the embattled Hamas group.
The internationally backed Palestinian President Mahmoud Abbas, whose forces lost control of Gaza to Hamas in 2007, angrily accused the US and Israel of being involved in a “conspiracy” to permanently sever Gaza from the West Bank.
He promised to take unspecified measures against his Hamas rivals in the coming days.
Hamas leaders in Gaza have described the arrival of the 15 million dollars, delivered last week in three suitcases by a Qatari diplomat, as their first major gain of more than seven months of weekly protests along the perimeter fence.
Hamas has been leading the protests since March 30 in a bid to ease a crippling Israeli-Egyptian blockade that was imposed in 2007 in order to weaken the militant group.
The blockade has led to over 50% unemployment and chronic power cuts, and prevents most Gazans from being able to leave the tiny territory.
Israel says it is defending its border against militant infiltrations, but its army has come under international criticism because of the large number of unarmed protesters who have been shot.