* U.S. expresses "grave concern" at situation
* Teenagers were travelling home from West Bank seminary
* Palestinian security official denies PA involvement (Adds comments from U.S. Secretary of State Kerry)
By Ori Lewis
JERUSALEM, June 13 (Reuters) - Israeli forces are searching for three Jewish teenagers who went missing in the occupied West Bank late on Thursday, the military said on Friday.
As media speculated that the three youths might have been abducted, large numbers of Israeli soldiers scoured the countryside around the flashpoint city of Hebron, carrying out house-to-house searches in neighbouring villages and blocking roads.
Local media said the three youngsters had last been seen trying to hitch-hike home from a religious seminary in the Jewish settlement of Gush Etzion, to the north of Hebron.
"Forces are conducting a widespread operation to locate the individuals," the military said in a statement.
Prime Minister Benjamin Netanyahu convened a special meeting of security ministers and said in a statement that Israel held President Mahmoud Abbas's Western-backed Palestinian Authority responsible for the safety of the three.
But Adnan al-Dmairi, a spokesman for Palestinian security services in the West Bank, deflected Israel's criticism.
"Three settlers are missing - why is this the fault of the Palestinian Authority? We have nothing to do with this issue. If a natural disaster hits Israel, would we be responsible? This is mad and unacceptable. We have no knowledge about this," he said.
The military did not name the teenagers. The newspaper Haaretz said two were aged 16 and one was 19. Local media added that one of the three also held American citizenship, and that the U.S. ambassador to Israel had been briefed.
U.S. Secretary of State John Kerry "expressed grave concern ... and ... our commitment to working with both the Government of Israel and the Palestinian Authority to try to ensure the situation is resolved quickly and the teenagers are returned to their families," a U.S. spokesman said.
"Secretary Kerry has ... spoken to President Abbas to urge him to do everything possible to assist in the effort to find them. President Abbas assured him that he is doing so."
Kerry met Israeli chief peace negotiator Tzipi Livni at a conference in London and later also spoke to Netanyahu, an Israeli spokesman said.
"The prime minister said to Kerry: Abu Mazen (Abbas) is responsible for the wellbeing of the missing (boys)," part of the Israeli statement about the conversation said.
Palestinian militants have said in the past that they want to kidnap Israelis to win concessions from the Israeli government. Some 1,027 Palestinian prisoners were freed in 2011 in return for the release of an Israeli soldier held captive in the nearby Gaza Strip for more than five years.
Chief military spokesman Brigadier-General Motti Almoz said security agencies were "making a very large intelligence effort to try to glean information on what happened to these three youths in the past hours".
In September 2013, an Israeli soldier was kidnapped and killed by a Palestinian who had lured him to the West Bank. Police say the kidnapper wanted to use the soldier to obtain the release of his brother, held in an Israeli jail. (Additional reporting by Ali Sawafta in Ramallah and Arshad Mohammed in London, Editing by Crispian Balmer, Kevin Liffey and Larry King)