A senior US defense official called Russian President Vladimir Putin’s “cavalier” language around the nuclear-capable missile systems pledged to Belarus “pretty irresponsible.”

“Our strategic forces are always monitoring things in that regard,” said the official in a background call with reporters. “We are certainly taking that seriously and have taken that threat seriously from the very beginning.”

Here's the full quote:

“Certainly any time anybody uses the word nuclear, you have concerns. Quite honestly it seems pretty irresponsible of a national leader to talk about the employment of nuclear weapons and to do so in a generally cavalier fashion. In terms of my concerns, other than the fact that they talk about, again, I mean the way that that statement read from Putin was, hey we’re going to give them Iskanders, and oh by the way they can hold nuclear weapons. And everybody takes that very seriously when you use that language. So we are certainly taking that seriously and have taken that threat seriously from the very beginning. And as you know our strategic forces are always monitoring things in that regard.”

Some context: Russia will transfer nuclear-capable Iskander-M missile systems to Belarus over the coming months, Russian President Vladimir Putin told Belarusian President Alexander Lukashenko at a meeting in St. Petersburg on Saturday.

“In the next few months, we will transfer to Belarus the Iskander-M tactical missile systems, which, as you know, can use both ballistic and cruise missiles, both in conventional and nuclear versions," Putin told Lukashenko, according to the Kremlin.

In a transcript of the meeting, Lukashenko expressed to Putin his "stress" and concerns over what he alleged are flights by the United States and NATO planes "training to carry nuclear warheads" close to Belarus' border.

Lukashenko asked Putin to consider “a mirrored response" to the flights or to convert Russia's Su-35 fighter jets, which are currently deployed to Belarus, so that "they can carry nuclear warheads."

Putin replied that although it is possible to match the US flights, "there is no need," and suggested that because Belarus' military has a large number of Su-25 aircraft that can be converted to nuclear-capable instead.

“This modernization should be carried out at aircraft factories in Russia, but we will agree with you on how to do this. And accordingly, start training the flight crew,” Putin said.

The Iskander-M is a Russian-built short-range ballistic missile system that can carry conventional or nuclear warheads with a maximum range of up to 500 kilometers (310 miles), according to Janes Defense.

The weapon uses both optical and inertial guidance systems to strike its targets, hitting them with a range of warheads, such as cluster munitions, vacuum bombs, bunker-busters, and electromagnetic pulse (EMP) warheads, according to the Missile Defense Advocacy Alliance. 

The Iskander-M was first used in 2008 during the Russia-Georgia conflict, when the Russian Army used it to hit targets in Gori, according to the alliance.

Mariya Knight and Jonny Hallam contributed to this report