Russia-Ukraine War: Why is Ukraine Firing American & British Missiles on Russia?

Ukraine has accused Russia of using an intercontinental ballistic missile (ICBM) for the first time in the war. If true, this would mark the first instance of ICBM use on an active battlefield. However, Western officials are skeptical of Ukraine's claims, and Russia has not commented on the matter. What is really happening? Is Ukraine exaggerating? Is Ukraine Trying to Provoke a War Between the U.S. and Russia?