The Israeli army has started calling up 16,000 reservists in what might be a preparation for a possible ground offensive into the Gaza Strip.

"As part of Operation Pillar of Defense, the IDF (army) will begin recruiting 16,000 reservists," Israeli the military said on its official Twitter feed.

Israel's Defense Minister Ehud Barak on Thursday approved the call-up of 30,000 reserve soldiers, who can be drafted into action by the military at any point, the army's official spokesman said, as cited by international media.

The move came as Israel pressed a relentless air campaign against Gaza militants which looked increasingly likely to expand into a ground operation after a rocket struck the sea just off the coast of Tel Aviv and a second landed to the south of the sprawling coastal city.

"We are in the process of expanding the campaign," the military's chief spokesman, Brigadier General Yoav Mordechai, told Channel 2 television on Thursday.

"The defense minister approved ... based on the army's request, the recruitment of another 30,000 soldiers. We will determine how many of them will be called in," he said.

Senior cabinet minister Moshe Yaalon also warned that Israel was considering a ground operation in order to stamp out rocket fire.

"We are preparing all the military options, including the possibility that forces will be ready to enter Gaza in the event that the firing doesn't stop," he wrote in a series of postings on his official Twitter account.