Thousands are feared dead after Iran’s violent crackdown on anti-government protests, as internet shutdowns make it difficult to verify the scale of casualties.

Iran has warned it will retaliate if attacked by the United States as protests spread across the country despite an intensified security crackdown. Human rights groups report hundreds killed and thousands detained amid an escalating crisis.

Protests in Iran have entered their 13th day as authorities impose a nationwide internet blackout and security forces clash with demonstrators in several major cities.