UNICEF’s spokesperson James Elder. File photo

The United Nations warned on Tuesday that it was impossible to create so-called safe zones for civilians to flee to inside the Gaza Strip amid Israel's bombing campaign.

Israel had initially focused its offensive on the north of the territory, but the army has now also dropped leaflets on parts of the south, telling Palestinian civilians there to flee to other areas.

"The so-called safe zones... are not scientific, they are not rational, they are not possible, and I think the authorities are aware of this," James Elder, spokesman for the UN children's agency UNICEF, told reporters in Geneva via video-link from Cairo.

His comments came as Israeli troops battled Palestinian group Hamas in the southern Gaza Strip after expanding their offensive deeper into the besieged area.

In retaliation for the worst attack in its history, Israel has vowed to eradicate Hamas and secure the release of all the hostages held in Gaza.

Agence France-Presse