US President Donald Trump said on Monday that before agreeing to provide Tomahawks, he would want to know what Ukraine planned to do with them because he did not want to escalate the war between Russia and Ukraine.
He said, however, that he had "sort of made a decision" on the matter.
Tomahawk missiles have a range of 2500km, meaning Ukraine would be able to use them for long-range strikes deep inside Russia, including Moscow.
Some retired variants of Tomahawks can carry a nuclear warhead, according to the US Congressional Research Service.
"The topic of Tomahawks is of extreme concern," Kremlin spokesman Dmitry Peskov told Russian state television reporter Pavel Zarubin in remarks published on Sunday.
"Now is really a very dramatic moment in terms of the fact that tensions are escalating from all sides."
Peskov said that if Tomahawks were launched at Russia, the Kremlin would have to take into account that some versions of the missile can carry nuclear warheads.
"Just imagine: a long-range missile is launched and is flying and we know that it could be nuclear. What should the Russian Federation think? Just how should Russia react? Military experts overseas should understand this," Peskov said.
Russian President Vladimir Putin said earlier this month that it was impossible to use Tomahawks without the direct participation of US military personnel and so any supply of such missiles to Ukraine would trigger a "qualitatively new stage of escalation".
The Financial Times reported on Sunday that the US has been helping Ukraine mount long-range strikes on Russian energy facilities for months.
The FT said US intelligence helps officials in Kyiv shape route planning, altitude, timing and mission decisions, enabling Ukraine's long-range, one-way attack drones to evade Russian air defences.
Zelenskiy told Fox News on Sunday that Ukraine would only use Tomahawk missiles to pursue military goals and not attack civilians in Russia.
