Supreme leader strikes defiant tone in remarks reported by state media, after not being heard of or seen publicly in days.

The U.S. Department of Homeland Security warned Americans of a "heightened threat environment" on June 22 that could include attacks on U.S. soil. That fear appeared to dissipate when Iran attacked a United States military base in Qatar June 23, injuring no one.

President Donald Trump on Monday announced that Israel and Iran had agreed to a ceasefire, after more than a week of fighting following Israel's strikes -- and subsequent U.S. attacks -- on Iran's nuclear program.