Ukraine claims Russia fired an intercontinental ballistic missile at the city of Dnipro on Thursday, in what would be the first use in war of a weapon designed to carry out long-range nuclear attacks.
Western officials, who spoke on condition of anonymity, later told Reuters that their initial analysis showed it was not an intercontinental ballistic missile, although they left open the possibility that the conclusion could change.
Ukraine fired US and British missiles at targets inside Russia this week, despite warnings from Moscow that it would see such a move as a major escalation.
Security experts said that if Thursday's attack involved an intercontinental ballistic missile, it would be the first use of such a missile in war. ICBMs are strategic weapons designed to deliver nuclear warheads and are an important part of Russia's nuclear arsenal.