Gaza City (AFP) - The spectre of a military invasion on Friday hung over Hamas-run Gaza Strip where two children were killed as Palestinian fighters fired more rockets despite Israeli threats of harsh retaliation. Two girls, one aged five and the other 12, were killed when their house in northern Gaza was hit by a rocket, which witnesses said was apparently fired by Palestinian fighters targeting southern Israel. Four other family members were wounded. The casualties came amid mounting speculation that the Israeli military would soon launch an operation in the Gaza Strip, which most media said would probably be limited in scope and not a full-scale invasion. "Army preparing for combined ground, air operation in Gaza," declared the front-page headline in Israel's Haaretz newspaper. Violence in and around the Palestinian enclave has flared since a six-month ceasefire ended on Dec 19, and escalated dramatically on Wednesday when fighters fired more than 80 rockets and mortar rounds after Israel launched deadly air strikes over the coastal strip. On Friday, 13 rockets and mortar shells hit southern Israel, causing no casualties but damaging a house that was unoccupied at the time. Israel had responded to earlier rocket attacks by tightening the blockade it has imposed since the Hamas violently seized power in Gaza in June 2007. But officials said dozens of truckloads of supplies were delivered to Gaza on Friday after Israel decided to temporarily allow humanitarian aid into the impoverished territory. At the same time, the Israeli government issued dire warnings to Gaza fighters, saying it would strike back hard if attacks continue. The movement " which is sworn to destruction of the Jewish state " has warned in turn that it would retaliate by resuming suicide bombings inside Israel. The last such attack claimed by Hamas was in January 2005. Popular pressure for a military operation has mounted in Israel. "The systematic shelling of civilians in Israel's communities is a war crime and a crime against humanity. The state of Israel has to protect its citizens," the mass-selling Yediot Aharonot said. Meanwhile, Iran's Red Crescent is sending 2,000 tonnes of food, medicine and appliances as well as 12 doctors and relief workers to Gaza in defiance of Israel's blockade, the Iranian state broadcaster said on Friday. Tehran is a staunch supporter of Hamas but rejects allegations it is supplying arms to the movement, saying it provides only moral backing. Pro-Palestinian activists, including European parliamentarians, had sailed to Gaza from Cyprus on five occasions over recent months, but Israeli forces turned back a Libyan ship seeking to deliver aid earlier this month.