UN Secretary General Ban Ki-moon has called on the Israelis and Palestinians to take urgent measures to try to end escalating violence in the Gaza Strip.
The rise in violence comes after US-led push for progress on peace
More than 30 people have been killed there in the past week.
He urged Israel to re-open crossings into the Strip and the Palestinians to halt rocket and sniper attacks.
On Friday a woman died in an Israeli missile strike on the Hamas-run interior ministry in Gaza City, following rocket attacks on Israel.
Mr Ban appealed to the Palestinians for "an immediate cessation" of sniper and rocket attacks from Gaza into Israel, and for "maximum restraint" on the part of the Israeli military.
He said closures cut off the people in the Gaza Strip from much-needed fuel supplies used to pump water and generate electricity to homes and hospitals.
"We all understand the security problems and the need to respond... but collective punishment of the people of Gaza is not, we believe, the appropriate way to do that," said John Holmes, undersecretary-general for humanitarian affairs.
Ministry building hit
A Palestinian woman was killed and more than 40 people were injured when an Israeli missile struck the Hamas-run interior ministry headquarters in Gaza City on Friday, medical sources say.
The building was vacant, having been severely damaged by an Israeli air strike in July 2006, but was situated in residential neighbourhood.
Hamas has launched over 100 rockets into Israel in recent days
A wedding was taking place nearby and many of those hurt were women and children, correspondents say.
Another Israeli air strike targeted a disused Hamas security forces building. No casualties were reported.
In a separate incident, a militant from the al-Aqsa Martyrs Brigades, a group linked to the Fatah faction of Palestinian Authority President Mahmoud Abbas, was also killed by Israeli troops in the West Bank town of Nablus.
Ahmed Senakreh was killed in a gun battle with soldiers who had surrounded a house in the Balata refugee camp where he was hiding.
The Israeli military has intensified operations in Gaza this week, killing at least 32 Palestinians, most of them militants, in air strikes.
Hamas, the militant Islamist group, has fired salvoes of unguided rockets for the first time in months, causing injuries in Israel.
The sharp rise in violence in the Gaza Strip comes after a recent US-led push for progress in peace talks.
Roads dug up
Israeli Defence Minister Ehud Barak took the decision to close Gaza's borders on Thursday evening, prompted by the rocket attacks.
More attacks followed on Friday, with one hitting the Israeli town of Ashkelon. No casualties were reported.
The Israeli military said more than 160 projectiles had fallen on Israel since Tuesday, the Associated Press reported.
Gaza receives humanitarian and fuel supplies through Israel
BBC Jerusalem correspondent Katya Adler says the closure should be seen as a warning by Israel to show what it can do if it decides to put increased pressure on Gaza.
The crossings would normally be closed from Friday afternoon to Sunday morning for the Sabbath. Israeli officials have indicated the decision will be reviewed on Sunday.
But, a spokesman for the United Nations Relief and Works Agency (UNRWA), Christopher Gunness, said it was unable to deliver lorry loads of aid because of the closure.
"Gaza is completely shut down. This will only add to an already dire situation," he said.
Gaza receives its fuel and humanitarian supplies through Israel, and the territory has been under an ever-tightening blockade since Hamas took it over by force in June.
Last week, a hopeful US President George W Bush was predicting the recently relaunched peace process could result in a deal within a year.
But on Thursday, the Palestine Liberation Organisation (PLO) headed by Palestinian Authority President Mahmoud Abbas warned of "serious consequences" for the talks if the Israeli strikes continued.