Iran's Revolutionary Guards rejected a US proposal for a emergency line.
The same day Iran launched strikes on American bases in Bahrain and Kuwait.
Both Kuwait and Bahrain acknowledged the attacks.
US officials said the strikes hit facilities used by American personnel.
The Iranian attack came shortly after the hotline offer was made.
President Donald Trump cautioned that Iran's existence as a state will cease to exist.
He pledged additional military action against Tehran.
Trump said any any further aggression would be met with decisive force.
The rejection of the hotline and the fresh attacks heighten the risk of wider conflict.