U.N. Secretary-General Ban Ki-moon urged Israel and Hamas to "stop this dangerous escalation," a U.N. spokesman said.
He described rocket attacks as "unacceptable" and urged "maximum restraint" by Israel.
Today's bombing in Tel Aviv is reminiscent of six years ago and could mark a serious escalation in the conflict.
A truce in Gaza suits all sides but none of the groups seem interested in a long-lasting solution.
A complex system of air defences has been instrumental in protecting Israel's population from incoming rockets.