Israeli tanks and troops massed outside Gaza and the military said on Friday it was calling up 16,000 reservists, signs of a possible imminent invasion of the Palestinian enclave after 48 hours of air strikes.

Israel's warplanes, drones and helicopters appeared to shift focus from suspected Palestinian rocket sites to the northern Gaza frontier, where their bombs created incursion corridors by clearing landmines or guerrilla gun nests.

The scale of the potential mobilisation prompted one commentator on Israel's Army Radio, who half-joked on air that so many troops would risk "falling over each other" in Gaza.

Interviewed by the station, Vice Premier Silvan Shalom was asked if Israel was preparing the military for possible flare-ups on other fronts, such as Lebanon, which the Jewish state has watched with concern given conflict in neighbouring Syria and furore over the nuclear ambitions of Hezbollah's patron Iran.

"We are taking everything into consideration," Shalom said, without elaborating.