Two Israeli traffic policemen have been shot dead near the West Bank town of Nablus and an operation is under way to try to catch the attacker or attackers.
The dying officers were found in their overturned car in the Massua district of the Jordan Valley.
It is the first fatal shooting of an Israeli in the West Bank since April of last year.
A previously unknown group, named after a dead Lebanese Hezbollah leader, has claimed responsibility for the attack.
The Imad Mughniyeh Group, named after the man killed by a bomb in Damascus just over a year ago, made its claim in messages to foreign media.
Israel denied any role in the Hezbollah leader's assassination.
The Israeli intelligence services had issued an alert saying there could be retaliation on the anniversary of his death, 12 February.
However, it was thought that this would target Israeli businessmen or holidaymakers abroad, the BBC's Paul Wood reports from Jerusalem.
Israeli police spokesman Micky Rosenfeld said a "nationalistic motive" appeared to have been behind Sunday's attack.
Our correspondent says the West Bank had become relatively peaceful before Sunday's attack.
Palestinian police, trained and funded by the West, have become increasingly effective in clamping down on militants, our correspondent says.
The attack took place in an area close to the Jordanian border and under Israeli security control.
Our correspondent says anger has been running high since Israel's offensive in the Gaza Strip, which left some 1,300 Palestinians dead.
The shooting also comes at a particularly sensitive time as Israel's government agonises over how many Palestinian militants it could release from prison in exchange for captured Israeli soldier Gilad Shalit, our correspondent adds.
Massua is located in the Jordan Valley, just south-east of Nablus and near the major West Bank settlement of Ariel.
The Haaretz newspaper says that a driver from the West Bank settlement of Kochav Hashahar was seriously wounded when a gunman opened fire on his car on the highway near the settlement in January.