JERUSALEM (AFP) -- Israel holds the Palestinian Authority responsible for the safe return of three teenagers kidnapped in the West Bank by a "terror organization," Prime Minister Benjamin Netanyahu said.
"Our young people have been kidnapped by a terror organization ... there is no doubt about that," Netanyahu told reporters in Tel Aviv as a massive manhunt for the three youngsters entered its third night.
The youngsters, one of whom also holds a US passport, are believed to have been snatched Thursday night from the Gush Etzion settlement bloc between Bethlehem and Hebron, reportedly while hitchhiking. They were studying at two Jewish seminaries in Jewish settlements in the southern occupied West Bank.
Their disappearance has prompted a huge search by the army and border police.
Netanyahu said he placed responsibility for their safe return on the shoulders of Palestinian president Mahmoud Abbas and his Palestinian Authority.
The Israeli leader demanded "the Palestinian Authority, headed by Abu Mazen (Abbas) -- from whose territory the kidnappers came -- do whatever necessary to help the hostages get home safely. That is the Palestinian Authority's responsibility."
The three youths were identified on Saturday as Gilad Shaer, 16, from Talmon settlement near Ramallah, Naftali Frenkel, 16, from Nof Ayalon in Israel, and Eyal Ifrach, 19, from Elad near Tel Aviv.
PA responsible for 'every attack'
The suspected abductions occurred 10 days after a new Palestinian unity government was sworn in, pieced together in agreement with the Islamist Hamas movement, in a development roundly denounced by Israel.
Israel has vowed to boycott all contact with the new government, whose emergence has ended seven years of divided rule between the West Bank and Gaza, with Netanyahu insisting Abbas be held responsible for all acts of violence emanating from anywhere in the Palestinian territories.
"We see Abu Mazen and the Palestinian Authority as responsible for every attack on Israel emanating from their territory, whether it is from Judaea and Samaria, or from the Gaza Strip."
He also said an April reconciliation agreement between Palestinian leaders in the West Bank and the Hamas movement in Gaza had "brought about the most serious consequences" and had "encouraged" groups committed to Israel's destruction.
Earlier, a spokesman for the Palestinian security services rejected Israel's accusations they had failed to prevent the kidnapping, saying the incident occurred in a settlement bloc in area C of the West Bank -- an area under full Israeli civilian and security control.
"Even if there was an earthquake, Netanyahu would blame the Palestinian Authority," General Adnan al-Damiri told AFP, describing Netanyahu's suggestions as "mad."
But Netanyahu dismissed Damiri's remarks outright.
"The claim that the Palestinian Authority is not responsible because the incident took place in an area under Israel's responsibility is completely unfounded," he said.
"The authority which controls the area from which those terrorists came is responsible. That is exactly the situation in this case. The terrorists came from the Palestinian Authority's area, and the PA is responsible for this."
As night fell, searches continued across the West Bank, focusing on the southern city of Hebron.
Earlier, a senior military officer said the three were presumed kidnapped by Palestinian militants but offered no suggestion as to who was responsible, adding substantial reinforcements had been drafted in.
"Tens of Palestinians" were arrested, and the Palestinian security forces were helping, he said.
A Palestinian official confirmed its security services were "cooperating" with Israeli agencies to gather information on the teenagers' disappearance.
"Because we have no information to the contrary, we are assuming that they are still alive," Defense Minister Moshe Yaalon told reporters.
Netanyahu said he had ordered the top military and security brass to use "all tools at their disposal to find the hostages, and secondly, to prevent their transfer to Gaza or any other place."
Troops had earlier closed the main crossings into Gaza to prevent the youths from being taken into the territory where the Islamist Hamas movement remains dominant despite resigning its authority following the formation of the unity government.
On Saturday, Israel launched air strikes on Khan Younis and Rafah in Gaza after militants fired a rocket over the border. Another three struck open areas in the afternoon, causing no injuries or damage but provoking yet more Israeli air raids.
As the scope of the abduction crisis emerged, US Secretary of State John Kerry telephoned Netanyahu and Abbas on Friday, sources on both sides said.
Ma'an staff contributed to this report.