GAZA CITY, Gaza Strip - Israeli helicopters attacked a Hamas training field in Gaza City early today, killing at least 14 militants and wounding 30 others in one of the deadliest air strikes on militants in four years of violence.

The attack came a week after Hamas suicide bombers blew up two buses in the Israeli city of Beersheba, killing 16 people.

Hamas vowed revenge for today's air strike. Hours later, Palestinian militants fired rounds of mortars and homemade rockets at Jewish settlements in the Gaza Strip and Israeli towns bordering the coastal area. One Israeli in the border town of Sderot was lightly wounded in a rocket attack, rescue officials said.

The army said it struck a field Hamas used to train militants for firing mortars and rockets. In the past month, Hamas assembled a large bomb and a suicide bomber's explosives belt at the training camp, the army added.

In Gaza, children stayed home from school today and shops remained closed in a sign of mourning. Black smoke billowed over the city as students burned tires.

During the past four years of fighting, Israel has frequently targeted Palestinian militants through air strikes and other military operations.

But the strikes are usually aimed at very specific targets - usually senior members of the group or militants on their way to an attack.

Today's attack was the first time the army had targeted such a large gathering of militants.

This policy, which Israel calls "targeted killings," has enraged the Palestinians and drawn international condemnation, especially because of frequent civilian casualties.

In today's attack, at least five helicopter missiles pounded the Hamas camp just after midnight.

Hundreds of angry Palestinians, many of them members of the Hamas military wing with blood on their clothes, gathered outside, shouting "Revenge, revenge."

"This bloody crime is a new wave of aggression committed against our people and against our sons," said Hamas spokesman Mushir al-Masri.

Palestinian officials said all 14 of the dead were Hamas members.