By Nidal al-Mughrabi, Fadi Shana and Henriette Chacar GAZA/JERUSALEM (Reuters) -The United Nations said on Wednesday that Israeli tanks struck a huge U.N. compound in Gaza sheltering displaced Palestinians, causing “mass casualties”, but Israel denied its forces were responsible and suggested Hamas may have launched the shelling. The attack, which the U.N. said hit a vocational training centre housing 30,000 displaced people in Khan Younis, southern Gaza’s main city, prompted rare outright condemnation from the United States. “Mass casualties have taken place, some buildings are ablaze and the…