Thirteen Palestinians were killed and 25 wounded last night when the Israeli air force attacked a field in the eastern part of Gaza City.
The Israeli army said the attack targeted an area used by militant group Hamas. The Israeli statement listed events that it said took place at the field in recent days, including assembly of a large bomb and a suicide bomber's vest, practice in hijacking vehicles, and training in preparing and firing mortars and rockets.
A Hamas spokesman, Mushir al-Masri, said: "This bloody crime is a new wave of aggression committed against our people and against our sons. It's an ongoing war: one day for us and one day for them."
Hamas claimed responsibility for a double suicide bombing last Tuesday in Israel's south that killed 16 Israelis. The bombers came from the West Bank city of Hebron.
Mr Masri denied that the targeted field was used for training, saying that it was a summer camp for Palestinian youth. Witnesses said, however, that all the casualties were members of the Hamas military wing or supporters.
Hospital sources said all of the dead were Hamas members, but that the wounded included civilians. The field is in the Shajaiyeh section of Gaza City, a known stronghold of Hamas.
It was the bloodiest incident in Gaza City in several months. Israeli forces often target Palestinian militants in Gaza in air strikes and other operations. In a week of heavy fighting in May 31 Palestinians and 13 Israeli soldiers were killed.
With smoke rising from the field, Palestinians were seen searching the blacked-out area with candles and flashlights, looking for victims.
Witnesses said a group of men gathered in the field about half an hour before the first two explosions, which caused most of the casualties, were heard. The other three blasts went off as people were running away, they said. Israeli helicopter gunships were seen in the area.
Ambulances and cars brought casualties to Shifa Hospital in Gaza City. Juma Shaka, a hospital spokesman, said 13 people were killed and 25 wounded, three critically. He said an earlier report of 14 dead was explained by the number of unidentified body parts brought to the hospital.
Israeli leaders had vowed to strike back hard in retaliation for last Tuesday's twin bombings in Beersheba, the first suicide attacks in the Jewish state in nearly six months.
Hamas, which is sworn to Israel's destruction, has been the main militant group spearheading the campaign of suicide bombings against Israelis during a nearly four-year-old Palestinian uprising.Reuse content