A Palestinian toddler has been killed and several other people wounded in a fire suspected to be carried out by Jewish extremists.
Israeli Prime Minister Benjamin Netanyahu condemned the attack, describing it as terrorism.
The assailants entered the village of Kafr Duma, near the West Bank city of Nablus, torching two houses and scrawling Hebrew graffiti, including "revenge" and "Price Tag" - a slogan used by Israeli extremists.
They threw Molotov cocktails, or fire bombs, at the houses, the Israeli military said.
The 18-month-old child, identified as Ali Dawabsheh, was killed, while his four-year-old brother and their parents were badly wounded and taken to hospital, Palestinian officials said.
The house was destroyed by the fire, apparently as the family slept inside, while a neighbouring home was badly damaged, but was empty and there were no casualties there.
Mr Netanyahu said he was "shocked over this reprehensible and horrific act".
"This is an act of terrorism in every respect. The state of Israel takes a strong line against terrorism regardless of who the perpetrators are," he said in a statement.
The Israeli military said it sent troop reinforcements to the West Bank, fearing the incident could trigger unrest.
The "Price Tag" group has been blamed for a series of mosque fires in the West Bank in the past few years, often in retribution for Israeli actions against illegally built settlements.
Israel tore down two illegal structures in the Beit El settlement earlier this week.
A spokesman for Palestinian President Mahmoud Abbas said Israel was responsible.
"Such a crime would not have occurred if the Israeli government did not insist on pursuing settlements and protecting settlers," Nabil Abu Rdainah said in a statement on the official Palestinian news agency WAFA.
It was the worst attack by Israeli extremists since a Palestinian teenager was torched to death in Jerusalem a year ago.
That followed the kidnapping and killing of three Israeli teenagers by Palestinian militants in the West Bank.
The arson attack comes a day after after six people taking part in an annual Gay Pride march in Jerusalem were stabbed by an ultra-Orthodox man in Jerusalem.
