Israeli aircraft launched a massive air raid on Hamas compounds inside the Gaza Strip on Saturday, killing at least 18 Palestinians, an official with the Islamist group and a medic said.

The warplanes launched simultaneous attacks on at least 30 separate targets in Gaza City.

A Palestinian medical official said at least 18 Palestinians were killed while Hamas officials said dozens were wounded.

Television pictures showed palls of smoke hanging over the city and wounded people lying on the ground.

Israeli leaders had threatened to carry out a major operation into the Islamist-controlled territory in a bid to halt rocket and mortar attacks by Palestinian militant groups after a six-month truce ended on December 19.

Palestinian medics said earlier three militants were wounded in the north of the densely populated and besieged strip by Israeli shellfire, but a military spokesperson in Tel Aviv denied involvement.

"At least two mortar rounds fired towards Israel landed in the northern Gaza Strip near the Erez crossing" with the Jewish state, he said.

The Ezzedine al-Qassam Brigades, the armed wing of Hamas, said in a statement that its men had fired four mortar rounds at Israel.

AFP