A huge blast late Wednesday rocked a training base run by the Islamic militant Hamas in southern Gaza, and at least five people were hurt, witnesses and a health official said.

Ambulances raced to the scene of the blast late Tuesday, on the site of an evacuated Jewish settlement next to the southern Gaza city of Khan Younis. The explosion could be heard all over the city.

It was not immediately known what caused the blast.

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.

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.