Israel has launched air raids against at least seven targets in the Gaza Strip, killing three people, Palestinian medics say.

The casualties on Thursday occurred near Gaza's border with Egypt where Israeli jets bombed two tunnels, which Palestinians use to smuggle goods into the blockaded coastal strip, medics and Hamas security officials said.

Palestinians said the body of a 14-year-old boy was recovered from one of the tunnels.

The air raids came after Israel said a dozen mortar bombs and rockets were fired from the Hamas-run territory into Israel. No casualties were reported from those attacks.

Israeli aircraft also hit three targets near Gaza City and two near the southern town of Khan Younis, striking empty buildings and open spaces, according to witnesses and Hamas officials.

Israeli media reports said Israel had targeted a weapons factory in Gaza City but the military had no immediate comment.

Al Jazeera's Ayman Mohyeldin, reporting from Gaza, said since Israel's war on Gaza ended last January, the Israelis have responded to mortar attacks from the Palestinian side with air raids on various parts of the Gaza Strip.

On Thursday, Israeli jets dropped thousands of leaflets over northern Gaza and Gaza City ahead of the attack, warning many of the 1.5 million residents to stay clear of the heavily-secured border, after the mortars and rockets were fired into Israel.

The leaflets, in Arabic, said anyone within 300m of the border "will be in danger".