TEL AVIV — Secretary of State Antony Blinken arrived in Tel Aviv on Thursday in a show of support for Israel, where he will tackle the crisis of unaccounted-for Americans who were killed or taken hostage by Hamas. Israel is massing troops near the Gaza Strip ahead of an expected ground assault, striking Hamas leaders after the militant group’s deadly cross-border attack. In Gaza, authorities warned of dire water and electricity shortages after Israel ordered a “full siege” of the densely populated, impoverished Palestinian enclave. The Palestinian Health Ministry in Gaza said Israeli airstrikes left hospitals at full capacity as medicine and fuel run out. Israel’s energy minister said “no electrical switch will be turned on” in Gaza until more than 100 Hamas hostages are freed.