* Three disappeared on Thursday near Jewish settlement
* Netanyahu puts onus on Abbas, whose forces help inquiry
* Palestinian official says Israeli leader wants "war"
The two 16-year-olds and a third, 19-year-old man 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 incident tests ties between Israel and Palestinian President Mahmoud Abbas, which were frayed by his power-sharing deal in April with Hamas Islamists hostile to the Jewish state.
In a televised statement delivered after he conferred with his security chiefs, Israeli Prime Minister Benjamin Netanyahu said the three teenagers had "definitely" been kidnapped - though he did not say by whom.
Netanyahu accused Abbas's alliance with Hamas of having emboldened Palestinian militants. He demanded the Palestinian leader do "all that is necessary" to resolve the crisis.
"The terrorists came from the (Palestinian) Authority's territory, and the Authority is responsible," Netanyahu said, referring to Abbas's administration, which exercises limited self-rule in the West Bank.
Wasel Abu Youssef, a senior member of Abbas's umbrella Palestine Liberation Organisation, rejected Netanyahu's charge.
"This is obfuscation meant to prepare for a military offensive as part of his ongoing and declared war against the Palestinian people," Abu Yousself told Reuters.
On Friday, the U.S. State Department said that, at its urging, Abbas was "working closely together" with Israel. Palestinian security officials said they were helping Israeli counterparts in the search. Hamas condemned that cooperation.
BOOTS ON THE GROUND
To escalate the searches and prevent the possible smuggling of the teenagers to the Gaza Strip or Jordan, Israel would bring a "significant" number of additional forces to the Hebron area, including a paratrooper brigade, a 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."
Without itself claiming responsibility, Hamas praised those behind the suspected kidnapping.
"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.
Netanyahu called off U.S.-brokered peace talks with Abbas over his reconciliation with Hamas, which controls Gaza 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 told Reuters.