* Netanyahu says efforts to retrieve youths may "take time," hints at expanded military operation
* Army chief says aims to hit hard at Hamas Islamist militants blamed for kidnappings
* Abbas condemns the abductions, Hamas ties straining (Updates with Netanyahu, spelling change due to abducted teen's family preference)
By Ali Sawafta
RAMALLAH, West Bank, June 16 (Reuters) - Prime Minister Benjamin Netanyahu prepared Israel for a long drawn-out operation to find three missing teenagers as troops expanded the search into a crackdown on the Islamist group accused of abducting them and arrested dozens of its officials.
Israel says members of Hamas, which signed a unity deal with Palestinian President Mahmoud Abbas in April, kidnapped the three seminary students, who disappeared on Thursday after leaving a Jewish settlement in the occupied West Bank.
Netanyahu said in televised remarks from a military base that the effort to retrieve the youths was a "complicated operation," and added "we must be prepared for the possibility it could take time."
He also said Israel had arrested more than 100 Hamas members in the West Bank and was "carrying out other actions." He did not elaborate.
"It is a serious event and it will have serious repercussions," Netanyahu said.
Lieutenant-General Benny Gantz, chief of Israel's armed forces, said the military was preparing to expand its operation.
"We have a goal, and that is to find these three boys and bring them home, and to hit Hamas as hard as possible - and that is what we are going to do," Gantz said at a meeting with army officers.
"We are on our way toward a significant campaign. We will get our plans in order."
Netanyahu, who broke off peace talks with Abbas after the Palestinian reconciliation pact, held a rare telephone conversation with the Western-backed Palestinian leader on Monday.
The prime minister's office said in a statement that Netanyahu told Abbas he expected him to help in efforts to find Gil-Ad Shaer and U.S.-Israeli national Naftali Fraenkel, both 16, and Eyal Yifrah, 19.
ABBAS CONDEMNS KIDNAPPINGS
In a separate statement, Abbas's office said the "Palestinian presidency condemns ... the kidnapping of three Israeli boys and the series of Israeli violations" - a reference to Israeli military raids and arrests.
Israeli officials have already cited security coordination with Abbas's Palestinian Authority in the search for the three.
Hamas, which advocates Israel's destruction, called such cooperation a "poisonous knife in the back of our people". Netanyahu has said Hamas members kidnapped the teenagers, which the group has neither confirmed nor denied.
Israeli troops conducted predawn door-to-door searches in six Palestinian towns on Monday, witnesses said.
The Israeli military said some 40 Palestinians, including "Hamas leadership and operatives", were arrested in the West Bank, raising to about 150 the number of people detained since Thursday.
Witnesses said several Palestinian lawmakers from Hamas, including parliament speaker Aziz Dweik, were taken into custody. The legislature has not convened since 2007 amid a rift between Abbas's Fatah movement and Hamas, which seized the Gaza Strip that year.
Most of the military efforts have been concentrated in the West Bank Palestinian city of Hebron, a Hamas stronghold.
In al-Jalazoun refugee camp, near the de facto Palestinian capital of Ramallah, Palestinians threw rocks at Israeli troops and army gunfire killed a 20-year-old Palestinian and wounded another, hospital officials said.
An Israeli government official, who spoke on condition of anonymity, said Israel was looking to leverage the search into a wider clampdown on Hamas in the West Bank and was also looking at legal aspects of deporting West Bank Hamas leaders to Gaza. (Writing by Maayan Lubell; Additional reporting by Nidal al-Mughrabi in Gaza; Editing by Jeffrey Heller and Sonya Hepinstall)