GAZA CITY, Gaza Strip ? Israeli helicopters attacked a Hamas training field in Gaza early today, killing at least 13 Palestinians and wounding 25, officials from both sides said. It was Israel’s deadliest strike in the area in four months.

The attack came a week after Hamas carried out a double suicide bombing in the southern Israeli city of Beersheba, killing 16 Israelis and breaking a six-month lull in major violence against the Jewish state. The suicide bombers were from the West Bank city of Hebron.

The Israeli military said the air force targeted the field, near the Israeli border, because it was being used by Hamas for bomb assembly and the training of anti-Israeli fighters. The Hamas military wing, in a statement, acknowledged its use as a training site.

Angry Hamas militants gathered at Shifa Hospital in Gaza City — some with blood on their clothes from carrying victims — shouting “revenge, revenge.”

“This bloody crime is a new wave of aggression committed against our people and against our sons,” Hamas spokesman Mushir al-Masri said. “It’s an ongoing war. One day for us and one day for them.”

The Israeli statement said training at the site “was led by senior Hamas terrorists who were involved in the carrying out of deadly terror attacks and attempted attacks.” The military said a suicide bomb prepared at the site was discovered last Tuesday in the underwear of a Palestinian at the Erez checkpoint between Gaza and Israel.

The airstrike was in the Shajaiyeh section of Gaza City, a known stronghold of the violence Islamic Hamas. The casualty toll was the highest in Gaza City since May, when a spate of heavy fighting killed 31 Palestinians and 13 Israeli soldiers.

On Monday, Israel’s defense minister Shaul Mofaz said he was moving another planned section of the West Bank separation barrier closer to Israel. Israel says it needs the barrier to keep out suicide bombers.

The barrier is part of Prime Minister Ariel Sharon’s contentious plan of “unilateral disengagement” from the Palestinians, including a withdrawal from the Gaza Strip and the evacuation of four small West Bank settlements in 2005.

Israel began construction of the southern segment of the barrier after Palestinian suicide bombers infiltrated across the unprotected line there and blew up two buses Tuesday in Beersheba.