Israeli Air Force fighter jets early Monday morning bombed sites in Gaza where Hamas is building terrorist tunnels, the military announced.

The attack was in response to two rockets launched at he southern Israeli city of Ashdod on Sunday night, the IDF said. The rockets landed in the sea; no damage or casualties were reported.

Though it was not clear which Palestinian terror group fired the rockets, the IDF said that “Hamas bears responsibility for what is happening in and out of the Gaza Strip,” and stated that it “takes all terrorist activity against Israel very seriously and is prepared and ready to act as resolutely as necessary against attempts to harm Israeli citizens and sovereignty.”

The event comes less than three weeks after Hamas and Palestinian Islamic Jihad launched a large-scale military drill involving 12 armed groups in Gaza, among them the Popular Front for the Liberation of Palestine and Fatah.

The drill, codenamed “The Strong Base” (a Koranic reference), involved the firing of rocket barrages into the Mediterranean and a simulated naval commando raid on Israel.

This was the first time that the Gaza-based terrorist groups’ “military” wings conducted a joint exercise of this kind, with each having its own role and field of operation.

The Dec. 29 drill came on the heels of IDF exercises along the Gaza border in southern Israel, and the Lebanon and Syria borders in the north. It took place three days after IAF jets hit several targets in Gaza in retaliation for two rockets fired at Israel on Dec. 25. Both projectiles, aimed at Ashkelon, were intercepted by the Iron Dome air-defense system.

A few hours before the drill began, just after midnight Monday, the IDF reported that terrorists in Gaza had fired a rocket at Israel, but that the projectile did not make it across the border.