The three boys disappeared on Thursday night after they left a Jewish settlement bloc where they were seminary students.
Since then the Israeli army has carried out house-to-house searches, round-ups and interrogations in the nearby Palestinian city of Hebron and outlying villages.
They arrested at least 12 Hebron area residents, including two women, confiscated video from privately owned security cameras and prevented around 300 residents from leaving the area, Palestinian officials and witnesses said.
The suspected kidnapping is a test for ties between Israel and Palestinian President Mahmoud Abbas, which were badly frayed by his power-sharing deal in April with Hamas, Islamists who are hostile to the Jewish state.
On Friday, the U.S. State Department said that, at its urging, Abbas was “working closely together" with Israel to resolve the crisis, drawing censure from Hamas.
A military source said Israel was working on the assumption that the missing three males - two of them 16 years old and the third a 19-year-old civilian - were abducted by Palestinians.
To escalate the searches, Israel was bringing a “significant" number of fresh forces to the Hebron area, including a paratrooper brigade, the military source said. “We need more boots on the ground to deal with this serious development. We need to be able to track them down, we need to use all of the capabilities at our hands in order to bring this to a quick end."
Defence Minister Moshe Yaalon told reporters that Israel’s “working assumption" was that the three were still alive. Yet the Israeli military source said: “We can’t confirm if they are alive or dead. We don’t know at this time."
Yaalon said Israel had foiled 14 attempts by Palestinians to abduct its citizens this year.
“It seems that this event got in under our radar."
HAMAS LAUDS “HEBRON HEROES"
A Palestinian security source said Abbas’s forces were helping Israel in the search, drawing condemnation from Hamas, which praised those behind the suspected kidnapping as “heroes".
“Security coordination between (Palestinian Prime Minister Rami) Hamdallah’s and Abbas’s security services and the enemy to locate the heroes of the Hebron operation and arrest them is a moral stain," said Hamas spokesman Fawzi Barhoum.
Palestinian militants have said in the past that they want to kidnap Israelis to win concessions from the Israeli government. More than 1,000 Palestinian prisoners were freed in 2011 in exchange for the release of an Israeli soldier held captive in the nearby Gaza Strip for more than five years.
In past deals with Lebanon’s Hezbollah militants, Israel swapped prisoners in return for the bodies of its slain troops.
A statement posted on social media on Friday said a Palestinian wing of the Iraq- and Syria-based Islamist militant group ISIL had seized the Israelis. The statement did not appear on regular ISIL websites, raising doubts as to its authenticity.
Hebron is a Hamas stronghold whose militants have at times eluded Israeli crackdowns. An Israeli soldier was killed while patrolling the city last September. The army described that as an attack by a Palestinian sniper who has yet to be captured.
Israeli Prime Minister Benjamin Netanyahu, who planned to convene his security cabinet after the Jewish sabbath ended on Saturday night to discuss the crisis, said he would hold Abbas’s administration responsible for the fate of the three.
Netanyahu called off U.S.-brokered peace talks with Abbas over his reconciliation with Hamas, which controls the Gaza Strip and has waged occasional shelling wars against Israel.
One of the three missing Israelis is a dual U.S. citizen, a person briefed on the investigation said. Reuters
Ali Sawafta and Nidal al-Mughrabi contributed to this story.