Israeli troops are battling Hamas militants in the center of the Gaza Strip’s second largest city, Khan Younis, the military says. The fighting has sent tens of thousands of Palestinians fleeing to the territory’s southernmost edge, where shelters are overflowing. Aid groups warn of a worsening humanitarian catastophe as fighting has prevented aid groups from delivering food, water and other supplies to the vast majority of Gaza's population. Khan Younis is now seeing the sort of fierce urban fighting and intensified bombardment that wreaked heavy destruction across northern Gaza.