GAZA—Several explosions rocked the Gaza Strip early on Monday, according to a Reuters witness, as Israel’s military said it attacked an underground site used by the Palestinian enclave’s Hamas Islamists to manufacture rockets.

The air strikes, in which there was no immediate word of casualties, followed what Israel described as its shooting down over the weekend of a rocket that had been fired over the border from Gaza. There was no Palestinian claim for that launch.

In the Israeli-occupied West Bank, another Palestinian territory, witnesses said troops had surrounded a house in the city of Nablus, with gunfire ensuing and possible casualties.

The Den of Lions, a group of Palestinian gunmen based mostly in Nablus and nearby Jenin, which has been subject to intensified Israeli raids over terrorist threats over the past year, said it had ambushed an army unit. Israel had no immediate comment.

Hamas extremists seized control of Gaza in 2007 and have fought several wars with Israel there since. When smaller Gazan factions attack Israel, it generally retaliates against Hamas.

Palestinian sources said Israeli ground forces also fired on Hamas border positions on Monday. Sirens sounded in Israeli towns near the Gaza border, warning of possible new rocket launches.