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Washington (CNN) - Rockets and shells crisscrossed the skies over southern Israel and Gaza Thursday.

Palestinian militants are continuing rocket attacks on Israeli civilians and Israel is pounding hundreds of what it calls "terror sites" in Gaza.

Israeli's military says it initiated "Operation Pillar of Defense" on Wednesday.

A U.S. State Department spokesman weighed in on the situation today.

"Let me be very clear that Israel has a right to self-defense. You know, as of yesterday, I believe the Israeli government noted that since the beginning of 2012, more than 768 rockets have been fired into Israel from Gaza and over 12,000 in the past 12 years and I believe over 100 today alone. They have a right to self-defense. The onus, let me be very clear, the onus here is on Hamas," said Deputy Spokesman Mark Toner.

Meanwhile, Defense Secretary Leon Panetta has spoken to Israeli defense minister Ehud Barak, according to a senior U.S. defense official.

It happened in the past few days as violence has escalated and Panetta has been traveling in Asia.