Four Palestinians have been injured early on Friday in a series of Israeli airstrikes in the Hamas- ruled Gaza Strip, medical sources and witnesses said.

The airstrikes came a few hours after Palestinian militants in Gaza fired four missiles into Israel.

Local witnesses said that Israeli warplanes hit right after midnight a number of military sites in the seaside enclave that belong to Palestinian armed factions and Hamas police.

Medical sources said four people were injured in the attacks and have been hospitalized.

One of the targets was a metal workshop in the northern Gaza Strip town of Jabaliya, according witnesses.

Israel has not commented on the strikes. But Israeli media outlets reported that four missiles hit open areas in southern Israel.

This is the first round of violence since the recent escalation between Israel and Gaza militants in mid-March when Israel killed three Islamic Jihad militants in Gaza, prompting the Islamist group to fire a barrage of missiles into neighboring Israeli communities.

In response to the rocket fire, Israel carried out several airstrikes that targeted military sites of the Islamic Jihad and Hamas movements.

The violence ended when Egypt restored a de-facto ceasefire it mediated in November 2012.