* Islamist group now caught up in Israeli arrest sweep
* Hamas armed strength in West Bank hard to gauge
* Israel, PA have kept Hamas under pressure in West Bank
By Ali Sawafta
RAMALLAH, West Bank, June 17 (Reuters) - The kidnapping of three Jewish teenagers may mark the emergence of Hamas as a force in the occupied West Bank after years under the thumb of both Israeli and Palestinian authorities.
Israel has been mounting a crackdown against members of the Islamist group in the territory, arresting more than 200, since the Jewish seminary students went missing near a settlement on Thursday.
Barak Ben-Zur, a former member of Israel's Shin Bet security service, said Hamas had thousands of armed operatives in the West Bank a decade ago, during a Palestinian uprising in which it carried out frequent suicide attacks in Israeli cities.
Now, he said, there are only a handful of militant cells in the territory, where an Israeli barrier now stands and the number of anti-Israeli attacks has dropped in recent years.
"But it is not about the number of terrorists, it's about the quality of attack. Look at what is happening now, one attack on three youths which has the entire political leadership of Israel dealing with that and nothing else," Ben-Zur told Reuters.
Hamas has neither confirmed nor denied Prime Minister Benjamin Netanyahu's allegations that it abducted the three seminary students, who went missing while hitchhiking near a Jewish settlement.
The group, which signed a unity deal with Western-backed Palestinian President Mahmoud Abbas in April, has been under constant security pressure in the West Bank from his Palestinian Authority since seizing the Gaza Strip in 2007.
Hamas has complained that dozens of its members have spent time in PA jails. Abbas's Fatah movement has said its activists in the Gaza Strip have been imprisoned by Hamas.
It also has been stung by a PA ban on its political activities in the West Bank and Israeli arrests in recent years of Hamas members of the Palestinian parliament, a forum that has not convened since the internal split seven years ago.
Some of the Hamas men detained by Israel in the past several days, including the speaker of parliament, have seen the inside of its jail cells before.
"Hamas's military might in the West Bank was never strong. They had sleeper cells, and no one has ever known their number, size or weaponry," said Nashaat al-Aqtash, a media studies professor at Bir Zeit University in the West Bank and a former consultant in Hamas's election campaign in 2006.
Experts said Hamas has its main West Bank strongholds in the cities of Hebron, Jenin, Qalqilia, Tulkarm and Nablus, and it is popular on Palestinian university campuses. On Tuesday, Israeli troops raided Jenin, a scene of bitter battles during the uprising a decade ago, to arrest activists.
But Reuven Erlich, head of the Meir Amit Intelligence and Terrorism Information Center near Tel Aviv, said that unlike Hamas's armed wing in Gaza - territory run by the Islamist group - its West Bank-based militants live deep in the shadows.
"One cannot talk about Hamas in the West Bank in military terms - size of forces, rockets and so on. If they have militant operatives, they are exposed only when caught," he told Reuters.
Erlich said that Hamas in the Gaza Strip, where the group's cross-border smuggling tunnels have been closed by a hostile leadership in neighbouring Egypt, is out to ignite a new uprising - "which is not happening" - in the West Bank.
"This shows two things: first, the relative weakness of Hamas (in West Bank) and second, the effectiveness of Israeli security forces and, to a certain extent, the effectiveness of Palestinian security," he said.
In past years, Palestinian security services in the West Bank have carried out aggressive campaigns to collect unlicensed weapons, confiscating large quantities particularly in Jenin and Nablus. PA coordination with Israel, in the search for the teenagers, was condemned by Hamas as a knife in its back.
But, Aqtash said, Hamas could eventually emerge from the current crackdown revitalised.
"Whoever reads history well discovers that the more Islamist movements are pressed, the stronger they get," he said.
(Reporting by Ali Sawafta in Ramallah, Nidal al-Mughrabi in Gaza and Maayan Lubell in Jerusalem, Writing by Jeffrey Heller; editing by Ralph Boulton)