The Russian government denied travel visas for Sens. Chris Murphy (D-CT) and Ron Johnson (R-WI) on Monday ahead of their scheduled trip to Russia with a bipartisan group of U.S. lawmakers.

In an official statement released on Tuesday, Murphy warned that the Kremlin’s decision is a “potentially perilous moment for our two nations’ fragile relationship.”

“As the owners of the world’s two largest nuclear arsenals with the capacity to destroy each other many times over, we have a responsibility to keep the world safe and prevent conflict between the U.S. and Russia,” the Democratic senator said. “Unfortunately, the Russian government is further isolating their country by blocking our visit and several others in recent months.”

Slamming the move as a “petty affront,” Johnson pledged to continue advocating a “strong and resolute response to Russian aggression” on Monday.

The Russian embassy responded by accusing Johnson of making “rude remarks” about Russian foreign policy “in his usual [R]ussophobic manner.”

We have long been calling on the #UnitedStates🇺🇸 to remove lawmakers from any travel restrictions as a first step towards abolition of «blacklists». #BTW the senator did not apply for a visa at our Embassy and did not inform about his plans to visit @Russia🇷🇺 https://t.co/SQLA4uvKPh pic.twitter.com/RQXBHozJg0 — Russia in USA 🇷🇺 (@RusEmbUSA) August 27, 2019

Murphy and Johnson, who both serve on the Senate Foreign Relations Subcommittee On Europe And Regional Security Cooperation, have pushed tough legislation against Russia in the Senate.