More than 500 Palestinian police reinforcements have been deployed in the city of Hebron in the West Bank.
The officers are patrolling Palestinian areas of Hebron, but not the area where Jewish settlers live guarded by Israeli troops.
Israeli officials say they approved the move to strengthen President Mahmoud Abbas's control of the occupied West Bank against the Hamas movement.
Hamas has accused Mr Abbas's forces of co-operating with the Israeli occupation.
Hamas seized control of the Gaza Strip in June 2007, ousting Mr Abbas's rival faction Fatah.
Since then, the Palestinian leader has intensified efforts to prevent Hamas taking over the West Bank, closing down Hamas-linked organisations and arresting supporters.
Lack of help could be costly
The United Nations says not a single country has responded to an appeal it made two weeks ago for urgent help for Syria, which is facing its worst drought in 40 years.
The UN says it needs over $US35 million to assist the one million Syrians it says are at risk.
Malnutrition is increasing and more than 100,000 people have lost at least half their livestock.