WASHINGTON, June 4, 2025 (BSS/AFP) - Ukraine's dramatic drone attack on Russian bombers over the weekend has increased the risk of escalation to "unacceptable" levels, said US envoy Keith Kellogg.

"The risk levels are going way up," Kellogg, President Donald Trump's envoy for Ukraine and Russia, told Fox News on Tuesday.

"When you attack an opponent's part of their national survival system, which is their nuclear triad, that means your risk level goes up because you don't know what the other side is going to do."

Ukraine conducted a massive drone attack against Russian military airbases on Sunday, saying it had destroyed several Russian nuclear-capable bombers worth billions of dollars.

The triad refers to a country's nuclear strike capabilities across land, air, and sea.

"But any time you attack the triad, it is not so much the damage done on the bombers... it's the psychological impact you have," said Kellogg, adding that it showed Ukraine "is not lying down on this."

The attack also showed Ukraine can raise risk levels "that are basically to me, they've got to be unacceptable."

Since Russia invaded Ukraine in February 2022, it has flexed its nuclear might to deter Kyiv's allies from intervening militarily.

Russia fired a medium-range Oreshnik missile at Ukraine in November that was not loaded but has the capability of carrying a nuclear warhead.

It is not the first time Ukraine has challenged Russia's strategic nuclear deterrence, having struck in 2024 an advanced radar system designed to warn Moscow of incoming ballistic missiles.