President Vladimir Putin says Russia would keep testing its new Oreshnik hypersonic missile in combat and had a stock ready for use, while Ukraine said it was already at work to develop air systems to counter the weapon.
Putin was speaking a day after Russia fired the new intermediate-range weapon into Ukraine for the first time, a step he said was prompted by Ukraine's use of US ballistic missiles and British cruise missiles to hit Russia.
Ukrainian President Volodymyr Zelenskiy said Kyiv was working with its Western partners to work out systems to counter "new risks".
Putin described the first use of Oreshnik (hazel tree) as a successful test, and said more would follow.
"We will continue these tests, including in combat conditions, depending on the situation and the nature of the security threats that are created for Russia," he said on Friday in televised comments to defence officials and missile developers.
"Moreover, we have a stock of such products, a stock of such systems ready for use."
A US official, however, said the weapon Russia used was an experimental one. The official said Russia has a limited number of them and that this is not a capability that Russia is able to regularly deploy on the battlefield.
Intermediate missiles have a range of 3000-5500km, which would enable them to strike anywhere in Europe or the western United States from Russia.
Security experts said the novel feature of the Oreshnik missile was that it carried multiple warheads capable of simultaneously striking different targets - something usually associated with longer-range intercontinental ballistic missiles designed to carry nuclear warheads.
Ukraine said the missile reached a top speed of more than 13,000km/h and took about 15 minutes to reach its target from its launch.
The firing of the missile was part of a sharp rise in tensions this week as both Ukraine and Russia have struck each other's territory with increasingly potent weapons.
Moscow says that by giving the green light for Ukraine to fire Western missiles deep inside Russia, the US and its allies are entering into direct conflict with Russia.
On Tuesday, Putin approved policy changes that lowered the threshold for Russia to use nuclear weapons in response to an attack with conventional weapons.
Zelenskiy, speaking in his nightly video address, has described Russia's use of the new missile as an escalation.
"On my behalf, the Minister of Defence of Ukraine is already holding meetings with our partners regarding new air defence systems capable of protecting lives from new risks," he said.
"When someone starts using other countries not only for terror, but also for testing their new missiles through acts of terror, then this is clearly an international crime."
Ukrainians, he said, had to remain vigilant.
"We must be aware that 'comrade' Putin will keep trying to intimidate us," he said. "That is how he built all his power."
The Kremlin said the firing of the Oreshnik was a warning to the West against taking further "reckless" actions and decisions in support of Ukraine.
The Oreshnik was fired with conventional, not nuclear warheads. Putin said it was not a strategic nuclear weapon but its striking power and accuracy meant that its impact would be comparable, "especially when used in a massive group and in combination with other high-precision long-range systems".
He said the missile could not be shot down by an enemy.
"I will add that there is no countermeasure to such a missile, no means of intercepting it, in the world today," he said.
"And I will emphasise once again that we will continue testing this newest system. It is necessary to establish serial production."
