A five-month truce has been under increasing strain in recent weeks
Israeli aircraft have fired missiles at targets in the northern Gaza Strip, injuring two Palestinian militants.
Militants fired rockets into Israel on Friday. One person in the town of Sderot was injured by shrapnel.
Rockets hit near the Israeli town of Ashkelon, 15km (nine miles) from Gaza. No injuries were reported.
On Thursday, the United Nations agency for Palestinian refugees said it had no more food to distribute in the Gaza Strip, because of the Israeli blockade.
A five-month truce between the two sides has been under increasing strain in recent weeks.
Israel says it will not re-open border crossings unless Palestinian militants stop firing rockets from Gaza.
Militants say the mortar and rocket fire is their response to what they say is Israeli aggression against Gaza.
John Ging, of the United Nations Relief and Works Agency (Unrwa), said on Thursday that handouts for 750,000 Gazans would have to be suspended until Saturday at the earliest, and called Gaza's economic situation "a disaster".
Israel earlier denied entry to a convoy carrying humanitarian supplies. It has prevented the transfer of all goods into Gaza for nearly a week, blaming continuing rocket attacks by Palestinian militants.
Unrwa distributes emergency aid to about half of Gaza's 1.5 million population.
Also on Thursday, Israel refused permission for a group of senior European diplomats to visit the coastal enclave.
It has also prevented journalists, including those from the BBC, from entering the territory.
Limited supplies of fuel were sent over on Tuesday after Gaza's only power plant ran out of diesel.
On Wednesday, Israeli troops killed four Palestinian militants from the Hamas movement, which has controlled Gaza since it wrested power from the Fatah-dominated Palestinian Authority in June 2007.
Witnesses said fighting broke out on the Gaza border after Israeli armoured vehicles crossed into the territory near Khan Younis.
The army said its soldiers were trying to stop militants plant a bomb near the security fence surrounding the strip.
Aleem Maqbool reports from a closed crossing into the Gaza Strip.
The Gaza power plant provides most of the electricity used in Gaza City; Israel supplies most of the rest of the territory's energy needs, but the system is liable to become overloaded and blackouts are common.
Israel occupied Gaza in 1967, but pulled military forces and Jewish settlers out in the summer of 2005.
Access to the territory remains under the control of Israel's military, as does its airspace and territorial waters.
Egypt controls the southern entrance to Gaza at Rafah, and goes along with the policy of isolating the Hamas movement, which Israel and its allies brand a terrorist group.
The current round of clashes and rocket fire began on 5 November when Israeli troops entered Gaza to destroy what Israel said was a tunnel dug by militants to abduct its troops.
One militant died in the gunfight, and a subsequent Israeli air strike on Hamas positions in southern Gaza killed at least five fighters.
Hamas responded with a barrage of rockets fired into Israel. There has been intermittent rocket fire since.
A truce between the two sides declared on 19 June had largely held. Both sides have accused the other of violating the truce, but maintain that they remain committed to it.
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