The head of Iran's Revolutionary Guards said Saturday that the Palestinians are "not alone" in their fight against Israel as they faced a second day of air strikes in Gaza.

"Today, all the anti-Zionist jihadi capabilities are on the scene in a united formation working to liberate Jerusalem and uphold the rights of the Palestinian people," Major General Hossein Salami said in a statement on the Guards' Sepah News website.

"We are with you on this path until the end, and let Palestine and the Palestinians know that they are not alone," he told the visiting leader of Palestinian militant group Islamic Jihad, Ziad al-Nakhala, during a meeting in Tehran.

Israel has said it launched a "pre-emptive" operation against Islamic Jihad on Friday to avert an imminent attack following days of tensions along the Gaza border.

A senior commander of the group was killed in an air strike on Gaza City on Friday, one of a wave of strikes on the territory.

The Gaza health ministry said a five-year-old girl was among 12 people killed by the Israeli bombardment. More than 80 others have been wounded.

Palestinian militants hit back with a barrage of rocket fire into Israel, in the territory's worst flare-up since a war last year.

Salami stressed that the Palestinian response showed "a new chapter" has begun and that Israel "will pay another heavy price for the recent crime".

"The Palestinian resistance is stronger today than in the past," he said, adding that the militant groups have found "the ability to manage major wars".

Iran's foreign ministry condemned Israel's "brutal attack" on Gaza.

President Ebrahim Raisi said Israel has "once again showed its occupying and aggressive nature to the world," according to a statement from his office.

Iran is a major backer of Islamic Jihad and Nakhala has met Raisi and other officials during his visit.

Israel has accused Iran of smuggling weapons to Palestinian militant groups in Gaza. In March last year, it said it had intercepted two Iranian drones laden with weapons for Gaza.