Gaza militants on Tuesday fired a rocket into Israel for the first time in three months, rattling a cross-border truce that has held since Israel's military offensive against the Hamas-run territory.
Israel closed Gaza's main cargo crossing until further notice, an apparent warning to Gaza's Hamas rulers to clamp down on rocket squads.
"Quiet will be met with quiet, missiles will be met with a response," said Israeli President Shimon Peres, adding that he believes both sides "have a deep interest in lowering the flames."
Militants claiming affiliation with the Al Aqsa Martyrs' Brigades, a violent offshoot of Palestinian President Mahmoud Abbas' Fatah, movement took responsibility.
They said in an email to journalists that they fired the rocket to avenge the death of a Palestinian in Israeli custody. It was impossible to independently verify the claim of responsibility.
The detainee, Arafat Jaradat, 30, died over the weekend after interrogation by Israel's Shin Bet security services. Palestinian officials, citing an autopsy, said the detainee was tortured, while Israel says more tests are needed to determine the cause of death.
U.N. political chief Jeffrey Feltman called for a independent investigation into Jaradat's death. He also called the rocket fire from Gaza "troubling" and appealed to Israel to "exercise maximum restraint."
Jaradat's death has sparked protests in the West Bank, including near the town of Bethlehem on Monday.
Two Palestinian teens, ages 13 and 16, were wounded in a confrontation with Israeli soldiers. The older boy was transferred to Jerusalem's Hadassah Hospital after being shot in the head and was in critical condition Tuesday, hooked up to a respirator, officials said.
The rocket from Gaza landed south of the Israeli city of Ashkelon early Tuesday, said police spokesman Micky Rosenfeld. The rocket caused damage to a road but no injuries, he said.
It was the first rocket fired from Gaza since Israel's military offensive against rocket squads in the coastal strip last November. The Hamas militant group has ruled Gaza since ousting forces loyal to Abbas in 2007.
Over the past decade, Gaza militants have fired thousands of rockets and mortar shells at Israel, and Israel has responded with military strikes. In between periods of cross-border violence, informal cease-fires have taken hold.
Hamas government spokesman Ehab Ghussein denied a rocket was fired, indicating Hamas was trying to distance itself from the incident. In the past, militant splinter groups have fired rockets at times when Hamas tried to discourage such attacks.
Hamas has enforced an Egyptian-brokered cease-fire that ended eight days of fighting with Israel in November, when Gaza militants fired hundreds of rockets at Israel as the Israeli air force pounded targets in Gaza.
Israel's closing of the Kerem Shalom cargo crossing with Gaza appeared aimed at pressuring Hamas to prevent further rocket fire. Kerem Shalom is the main conduit for goods into Gaza, though smuggling of goods from Egypt through tunnels continues.
Under the cease-fire, Israel pledged to increase the movement of goods into Gaza, which has been subject to an Israeli blockade since the Hamas takeover. Until Tuesday's closure, Israel was permitting most consumer items into the coastal strip.
A military statement said the crossing would be reopened only "following a government directive."
In the West Bank, meanwhile, Abbas accused the Israeli military of using increasingly harsh methods to clamp down on Palestinian rock-throwing protests. There has been an increase in clashes between Palestinian stone throwers and Israeli troops in recent weeks
"We don't want tensions. We don't want escalation," Abbas said Tuesday, rejecting recent allegations by Israeli officials that he was stoking tensions for political gains.
In Monday evening's confrontation near Bethlehem, the Israeli military said protesters threw "improvised hand grenades" at a Jewish shrine in the area, endangering worshippers inside.
Associated Press writer Edith M. Lederer contributed from the U.N.