HELENA — Russian nuclear inspectors visited the U.S. amid heightened tensions between the two nations to verify that 18 nuclear missile launch facilities had been demolished as part of a 2011 arms control treaty, Air Force officials said Monday.

The April 9 inspection — the first of its kind at Montana’s Malmstrom Air Force Base, according to treaty compliance chief Richard Bialczak  — went ahead despite the strain between the two nuclear powers over Russia’s intervention in Ukraine.

Vice President Joe Biden was in Ukraine on Wednesday to send a high-level signal of Washington’s support for Kiev. The U.S. has threatened additional sanctions against Russia if it does not heed an international agreement meant to de-escalate tensions.

Russia is allowed eight inspections of U.S. facilities each year under New START, or Strategic Arms Reduction Treaty, which is designed to reduce the number of deployed nuclear missiles by 2018.

Malmstrom, which is responsible for a third of the nation’s 450 intercontinental ballistic missiles, received notice of the inspection the day before the Russian teams and their escorts from the Defense Threat Reduction Agency arrived.

The Russian inspection teams spent 12 hours traveling to sites across central Montana to confirm that each silo’s launch doors had been removed and their launcher tubes filled with earth and gravel, Malmstrom Air Force base officials said in a statement.

“Overall, we felt the process went smoothly,” said Col. Marne Deranger, the vice commander of Malmstrom’s 341st Missile Wing.

The demolished launch facilities were operated by the 564th Missile Squadron, which was deactivated in 2008. Three other missile squadrons are responsible for the 150 intercontinental ballistic missiles at Malmstrom.

The remaining Minuteman III missiles are located at Minot Air Force Base in North Dakota and F.E. Warren Air Force Base in Wyoming.

The Russian inspectors could return to view 16 additional launch facilities at Malmstrom that were demolished after the inspectors departed, Air Force officials said. Eventually, all 50 of the 546th squadron’s former launch facilities will be demolished.

The U.S.-Russia treaty limits the number of strategic nuclear weapons each country can deploy to 700 by 2018. The Pentagon announced earlier this month it would keep all 450 land-based intercontinental ballistic missiles but remove 50 missiles from their launch silos to comply with the treaty.