At least 20 Palestinians and an Israeli soldier were killed in the Gaza Strip as Israel pressed an incursion aimed at stopping cross-border rocket fire. Most of the Palestinian fatalities were gunmen who attacked Israeli armored forces as they moved into northern Gaza on Thursday. At least three Hamas terrorists died in an Israeli airstrike on the southern town of Khan Yunis. An Israeli soldier, shot in the head by a Palestinian sniper as his unit entered the northern Gaza town of Beit Lahiya in search of rocket crews, later died of his wounds. Israeli officials said the buffer zone set up early Thursday is a stopgap measure in light of the expanding range of Kassam rockets fired by Hamas terrorists, which recently have begun falling on Ashkelon, a southern Israeli city.