JERUSALEM -- Recent Palestinian attacks on West Bank settlers, which are likely to increase in response to relaunched peace talks, pose one of the biggest challenges yet to U.S.-trained Palestinian security forces and their uneasy alliance with the Israeli military.

The militant group Hamas killed four Israeli settlers and wounded two others in shooting attacks this week, as Palestinian and Israeli leaders were starting their first direct negotiations in nearly two years.

With the talks hanging by a thread, an uptick in violence is the last thing the Palestinian Authority wanted to see, particularly because it could strengthen Israel's hand in negotiations. Israeli Prime Minister Benjamin Netanyahu hopes to make security the top issue.

Now, pressure is mounting on the Palestinian Authority to show that it can contain the violence quickly.

"This is the most serious threat they've faced," said Jonathan Fighel, a senior researcher at Israel's International Institute for Counter Terrorism and a former Israeli military governor in the West Bank. The last three years have been among the quietest on record in the West Bank, he said, but Hamas' announcement that it would resume armed attacks in the territory could lead to more instability.

Palestinian security forces, which have been reformed and retrained over the last three years, have made a strong show of force in the last few days. They have arrested hundreds of suspects in dozens of raids.

Success at keeping a lid on violence also depends in large measure on the Israel Defense Forces, which have ultimate authority over the entire West Bank and still control day-to-day security for 60 percent of West Bank land, including the sites of the two recent drive-by shootings near the cities of Hebron and Ramallah.

Although cooperation between Palestinian and Israeli forces has improved, friction and distrust remain. Palestinians complain that Israel doesn't share intelligence, limits access to weapons and makes raids into Palestinian towns without notifying Palestinian authorities.

Israelis say Palestinians aren't ready to take over security responsibility. Despite the increased training, they express reservations about the Palestinians' professionalism, noting that police used their guns against Israeli soldiers during the 2000 Palestinian uprising.

"Events on the ground will now compel the two sides to increase cooperation," Mr. Fighel said. "In light of the Hamas threat, Israelis need Palestinians and Palestinians need Israelis. They're in the same trench."

Israeli and Palestinian security officials say they are working together to identify and capture the militants responsible for the attacks. Analysts said the hundreds of arrests in recent days appeared to be aimed at sending Hamas a strong warning about how seriously the Palestinian Authority takes West Bank security.

Hamas split with the Fatah party of Palestinian Authority President Mahmoud Abbas in 2007, and now controls the Gaza Strip, where 1.5 million Palestinians live. Its operatives on the West Bank have moved underground. Palestinian forces arrested at least 370 people throughout the West Bank, mostly political activists, business owners and militants suspected of having ties with Hamas, an Islamist group that has been labeled a terrorist organization by the United States.

But Palestinian officials have complained that their ability to investigate this week's attacks and provide protection in the territories is limited because Israel retains exclusive control over Jewish settlements on the West Bank, nearly all major roads and large swaths of land it considers buffer zones.

The attacks against Israeli settlers "did not take place in territory under Palestinian security control," said Palestinian security forces spokesman Adnan Damiri. He said his forces should not be expected to take the lead in the case.

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