An off-duty Israeli soldier has been found dead with stab wounds near a Jewish settlement in the occupied West Bank.
Prime Minister Benjamin Netanyahu called it a "terrorist" attack.
Details were still emerging of the killing, but it risked raising Israeli-Palestinian tensions weeks ahead of 17 September Israeli polls.
It occurred between Bethlehem and the flashpoint city of Hebron in the West Bank.
"Today in the early morning hours, a soldier's body was found with stabbing marks on it adjacent to a (Jewish) community north of Hebron," the Israeli army said in a statement.
It later described it as a "terror attack" and identified the man killed as Dvir Sorek, 19. An army spokeswoman said he was not in uniform at the time.
Troops, police and the Shin Bet intelligence agency were searching the area, notably the nearby Palestinian town of Beij Fajjar.
The army said it had sent reinforcements to the West Bank.
Mr Netanyahu called it "a serious stabbing attack."
"Security forces are now in pursuit to capture the lowly terrorist and settle accounts with him," he said in a statement.
Later in a visit to the Israeli settlement of Beit El near Ramallah in the West Bank, Mr Netanyahu again spoke of the attack and pledged more settlement building.
The newly drafted soldier was a student at a yeshiva - or Jewish seminary - in the settlement of Migdal Oz, near where his body was found.
He was in a programme that combined military service with religious study, the seminary head told Israeli public radio.
"The soldier left for Jerusalem during the afternoon to buy a gift for his teachers," rabbi Shlomo Wilk said.
"He was in contact half an hour before he was murdered. He was on the bus to the yeshiva.
"About 100 metres from the bus stop, before he entered the settlement, he was murdered."
In Beit Fajjar, dozens of Israeli security personnel arrived in around 20 vehicles and were going house-to-house while seizing security camera footage, an AFP correspondent reported.
Low-level clashes broke out between residents and the Israeli forces.
Palestinian attacks against Israeli security forces and settlers occur sporadically in the West Bank, occupied by Israel since the Six-Day War of 1967.
Such attacks, and the Israeli arrest raids that follow, often stoke tensions.
Today's incident came at a sensitive time, with Israel heading towards a general election on 17 September. It also occurred just ahead of the Muslim Eid al-Adha holiday.
Mr Netanyahu is widely seen as wanting to avoid a major flare-up in either the West Bank or the Gaza Strip before the election, but he is likely to face political pressure to act firmly.
His main challenger, ex-armed forces chief Benny Gantz of the centrist Blue and White Alliance, spoke in stark terms.
"The (military) and Israeli security forces will know how to get their hands on these loathsome terrorists, dead or alive," he said in a statement.
Palestinian militant group Islamic Jihad welcomed the attack and called it a "natural reaction to the occupation's terrorism and crimes against the rights of our people, our land and our holy places."
Around 600,000 Israelis now live in settlements in the West Bank and annexed east Jerusalem next to some three million Palestinians.
Mr Netanyahu pledged in April to annex settlements in the West Bank, which would be a deeply controversial move.
Annexing settlements on a large-scale could be the death knell to already fading hopes for a two-state solution to the Israeli-Palestinian conflict.
Earlier this week, Israel advanced plans for more than 2,300 settlement homes, leading to firm criticism from the UN envoy for the Israeli-Palestinian conflict, Nickolay Mladenov.
He said settlement expansion was a "flagrant violation of international law" and called for it to "cease immediately and completely."