.GAZA CITY, Gaza Strip (AP) _ A huge blast rocked a training base run by the Islamic militant Hamas in southern Gaza late Tuesday, injuring at least five members of the group, witnesses and a health official said.

Ambulances raced to the blast site – an evacuated Jewish settlement next to the southern Gaza city of Khan Younis.

There was no immediate word on the cause of the explosion. Hamas declined comment, and the Israeli military said it had no knowledge of the incident.

Dr. Moaiya Hassanain of the Palestinian Health Ministry said two of the five wounded were in critical condition. He said all five were Hamas militants.

Witnesses said the blast cause heavy damage to the base, causing a fire that burned for an hour. They said a second, smaller blast was heard about 15 minutes after the first one. Hamas militants kept reporters away from the scene.

Israel and Hamas have been observing a truce that stopped Gaza rocket fire and Israeli reprisal raids. But in recent days there has been tension between Gaza’s Hamas rulers and their rivals from Fatah, which is led by moderate Palestinian President Mahmoud Abbas.