US defence secretary Pete Hegseth has warned that Tuesday would be the "most intense" day of strikes inside Iran, adding that the country "stands alone, and they are badly losing".
He also said that, in the last 24 hours, Iran has fired the lowest number of missiles since the start of the war.
On the ground, Iranians in Tehran have described how they were left "in total darkness" following heavy strikes overnight.
Meanwhile, Ali Larijani, a top Iranian security official, said his country is not scared of what he called "worthless threats" by US President Donald Trump.
During a Tuesday morning US defence department update on the operation in Iran, Hegseth said US goals include destroying Iran's missile stockpiles, their missile launchers and their defence industrial base; destroying Iran's navy; and permanently denying the country nuclear weapons.
