By Bassam Masoud and Nidal al-Mughrabi GAZA/CAIRO (Reuters) -Israeli tanks battled their way to the heart of Khan Younis on Sunday in a major new push into the main city of the southern Gaza Strip, as health authorities in Hamas-run Gaza said about 18,000 Palestinians had been killed in the war. Residents said tanks had reached the main north-south road through the middle of Khan Younis after intense combat through the night that had slowed the Israeli advance from the east. Warplanes were pounding the area west of the assault. The air rumbled with the constant thud of explosions and thick col…