Israeli Prime Minister Benjamin Netanyahu vowed to avenge what he said was a "black day" for Israel on Saturday, saying the army will strike back at Hamas in Gaza with full force.

"The IDF (army) is about to use all its force to destroy Hamas's capabilities," Netanyahu said in a brief televised statement.

"We'll strike them to the bitter end and avenge with force this black day they brought on Israel and its people," he said, after Hamas militants launched a surprise dawn attack, sending hundreds of militants into Israel under cover of a huge rocket barrage.