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The two 16-year-olds and a third man aged 19, seminary students in a Jewish settlement bloc, disappeared on Thursday (local time).
"These teenagers were kidnapped and the kidnapping was carried out by Hamas members," Prime Minister Benjamin Netanyahu told reporters, referring to the Palestinian Islamist group that controls the Gaza Strip.
There has been no public claim of responsibility. Asked about Netanyahu's allegations, Sami Abu Zuhri, a Hamas spokesman in the Gaza Strip, stopped short of a clear denial or confirmation that it was involved.
U.S. Secretary of State John Kerry, who had mediated peace talks that Netanyahu called off after Palestinian President Mahmoud Abbas agreed a power-share with Hamas in April, gave qualified backing to the prime minister's allegation.
"We are still seeking details on the parties responsible for this despicable terrorist act, although many indications point to Hamas' involvement," Kerry said in a statement.
"As we gather this information, we reiterate our position that Hamas is a terrorist organisation known for its attacks on innocent civilians and which has used kidnapping in the past."
Since the three teenagers vanished, apparently while hitchhiking, the Israeli army has carried out house-to-house searches, round-ups and interrogations in the Palestinian city of Hebron and outlying villages.
The army said it detained around 80 suspects overnight and would escalate the dragnet in the coming days. Palestinian officials put the number seized by Israel so far at more than 100, including at least seven Hamas members of the Palestinian parliament and several people recently freed from Israeli jails.
Israel identified the seminary students as Eyal Yifrach, Gil-ad Sha'er and Naftali Frankel, who also holds U.S. citizenship. In their last communication, one of the three managed to phone police on Thursday night to report that they were being kidnapped, according to an Israeli security official.
"TURNING THE WORLD UPSIDE DOWN"
"Naftali, your dad and mom and siblings love you endlessly, and you should know that the people of Israel are turning the world upside down to bring you home," Frankel's mother, Rachel, said in a televised statement outside the family home.
Thousands of Jews flocked to the Western Wall plaza in Jerusalem on Sunday evening to pray for the teenagers' return.
Gilad Erdan, a minister in Netanyahu's security cabinet, told Israel's Channel 2 television that Abbas's security forces were "willingly" helping search for the teenagers. Palestinian authorities acknowledged the cooperation, drawing Hamas censure.
Erdan played down, however, the role of a Palestinian administration which Netanyahu wants world powers to pressure into dissolving the Abbas-Hamas pact. Recovering the teenagers and dealing with their captors would be "almost entirely based on the Israeli military and security services," Erdan said.
In broadcast remarks at a cabinet session held at Israeli military headquarters in Tel Aviv, where he has been overseeing recovery efforts, Netanyahu warned of "serious consequences". Speaking later in English, he pledged: "Israel will act against the kidnappers and their terrorist sponsors and comrades".
Dismissing Netanyahu's "stupid comments", Hamas spokesman Abu Zuhri suggested the Israeli leader was trying to draw the group into disclosing whether it was behind the teenagers' disappearance.
Palestinian militants have said they want to kidnap Israelis to win concessions from the Israeli government, and the current incident coincides with a hunger strike by some 300 Palestinian prisoners protesting against detention without trial.
More than 1,000 Palestinian prisoners were freed in 2011 in exchange for the release of Gilad Shalit, an Israeli soldier held captive in the Gaza Strip for more than five years.
Netanyahu said Abbas's alliance with Hamas had emboldened militants in the West Bank, where the U.S.-supported Palestinian leader's Fatah movement has held sway, and demanded he do "all that is necessary" to resolve the crisis. Kerry said Washington "encouraged full cooperation between the Israeli and Palestinian security services. We understand that cooperation is ongoing."