GAZA CITY, Gaza Strip — Israel assassinated a Hamas strongman today in its first assault on the top leadership of Gaza's rulers, escalating a crushing aerial offensive even as it declared it was ready to launch a ground invasion.

The airstrike targeted the four-story apartment building that was home to 52-year-old Nizar Rayan, ranked among Hamas' top five decision-makers in Gaza. It also killed 12 other people, including two of Rayan's four wives and four of his 12 children, Palestinian health officials said.

The Muslim faith allows men to have up to four wives.

While intensifying its 6-day-old military offensive against Hamas in Gaza, Israel also appeared to be sounding out a possible diplomatic exit by demanding international monitors as a key term of any future truce.

Israel launched the offensive Saturday to crush militants who have been terrorizing southern Israel with rocket fire from Gaza.

More than 400 Gazans have been killed and some 1,700 have been wounded, Gaza health officials said. The U.N. says the death toll includes more than 60 civilians, 34 of them children.