The United Nations agency for Palestinian refugees says it is evacuating all its non-essential foreign staff from the Gaza Strip.
The announcement came as the Israeli army moved to expand a large-scale operation in northern Gaza.
The move aims to prevent Palestinian militants firing missiles into Israel.
In the latest violence, Israeli troops killed three Palestinian youths in Beit Lahiya near Gaza City, Palestinian officials said.
They were said to have been demonstrating against the Israeli operation.
Earlier, an Israeli helicopter fired a missile in the area, injuring at least two people, witnesses said.
An Israeli military source was quoted as telling the AFP news agency that the Israeli Air Force had fired into open fields near the Jabaliya refugee camp, north of Gaza City.
Elsewhere, a Palestinian bystander was killed on the West Bank during an exchange of gunfire in the city of Nablus between Israeli soldiers and Palestinians.
No safe route
Israeli tanks have moved to the entrance of the Jabaliya refugee camp - one of the largest in the area with about 45,000 people.
They were reinforcing tanks and infantry sent into the area last month to destroy farmland Israel says was being used as cover by Palestinian militants firing makeshift rockets.
Overnight the army expanded this operation pushing further south.
Bursts of helicopter gunship fire have been heard through the morning.
All this is going on around the road that leads to the only crossing point between Gaza and Israel.
Anybody entering or leaving the Strip now has to pass through this unstable, militarised zone, says our Gaza correspondent Alan Johnston.
This obviously became a major security worry for the UN which has a number of staff who need to travel frequently, our correspondent says.
Johan Eriksson, spokesman for the UN agency for Palestinian refugees (Unrwa), said 19 staff would now be relocated to Jordan - with only nine foreign workers left in Gaza.
Mr Eriksson said that reports that Israeli forces had moved to the edge of the camp meant there was no longer a safe route between Gaza City and the Erez crossing point into Israel.
Unrwa has always had its headquarters in Gaza, but the worsening security situation here in recent weeks has made its work harder, says our Gaza correspondent.
A fortnight ago the agency temporarily relocated 20 international staff to Jerusalem.
The southern Israeli town of Sderot, some three kilometres (two miles) from the Gaza perimeter fence, has frequently come under fire from such rockets which killed two Israelis in late June.