Israeli air strikes hit four Hamas sites in Gaza overnight, the military said Thursday, May 5, following earlier raids as tensions flared along the border of the Palestinian territory, AFP reports.

The spike in cross border violence has raised concerns for a ceasefire that has held since the last round of hostilities in Gaza ended in summer 2014.

"Overnight, in response to the ongoing attacks against Israeli forces, an IAF aircraft targeted four Hamas terrorist infrastructure sites in the northern Gaza Strip," the military said in a statement.

It did not mention if there were casualties.

The Israeli army hit five other Hamas operated sites earlier Wednesday in the south of Gaza, saying the raids were in response to mortar shells fired at its territory, AFP says.

Those strikes hit around Gaza's derelict international airport near the southern city of Rafah and in nearby farming areas, according the territory's Hamas-run interior ministry.

Israeli tanks also shelled the Palestinian enclave at least twice on Wednesday, while the army established a closed military zone around the small border town of Nahal Oz.

There were no reported casualties from those raids either, and no claims of responsibility for the mortar fire that hit Israeli territory.

But both Hamas and Islamic Jihad, another militant Palestinian group, released statements warning Israel against any escalation along the border.

Hamas' armed wing, the Ezzedine al-Qassem Brigades, said it was ready to respond to Israel's strikes.

"We will not permit this aggression to continue and the enemy should not invoke any reason and leave the Strip immediately," the group said.