Factbox-How Russia-US tensions have undermined key arms treaties

By Mark Trevelyan

(Reuters) - Russia's formal withdrawal from a landmark arms treaty on Tuesday is the latest example of the crumbling of the security architecture that was set up to make the world safer at the end of the Cold War.

Arms control pacts in the 1980s and 1990s were a sign of easing tensions and increased trust in East-West relations. Now the opposite is the case: Russia's invasion of Ukraine has brought about the deepest crisis in ties between Moscow and Washington since the 1962 Cuban missile crisis, with the Kremlin describing relations as "below zero".

Here is a list of the most important treaties and their current status.

INTERMEDIATE-RANGE NUCLEAR FORCES (INF) TREATY

U.S. President Ronald Reagan and Soviet leader Mikhail Gorbachev agreed in 1987 to scrap an entire category of nuclear weapons by permanently eliminating all missiles with ranges between 500 and 5,500 km (310 and 3,400 miles).

From 2014 onwards, the United States repeatedly accused Russia of deploying a non-compliant missile, which Moscow denied. President Donald Trump said in 2018 that the United States would therefore terminate the agreement, and Washington formally withdrew the following year.

Russia said it too would suspend its INF treaty obligations following the U.S. move.

CONVENTIONAL ARMED FORCES IN EUROPE (CFE) TREATY

Signed by 22 countries in 1990, the treaty set equal limits on the amount of weaponry, including tanks, heavy artillery and combat aircraft, that NATO and the Soviet-led Warsaw Pact could deploy between the Atlantic Ocean and the Ural mountains.

Russia formally withdrew on Tuesday but had not actively participated since 2015. Moscow said the U.S. push to enlarge NATO had led to alliance members "openly circumventing" its restrictions; NATO allies condemned Russia's withdrawal and said they would suspend their own treaty obligations.

COMPREHENSIVE NUCLEAR TEST BAN TREATY (CTBT)

The CTBT, agreed in 1996, bans "any nuclear weapon test explosion or any other nuclear explosion" anywhere in the world, with the goal of reducing and ultimately eliminating nuclear weapons. Russia ratified it in 2000 but President Vladimir Putin last week signed a law revoking that decision, arguing that Russia should mirror the position of the United States which has never ratified the pact. Russia says, however, that it will keep supplying data to the treaty's verification regime and will not conduct a nuclear test unless the United States does.

NEW START TREATY

Signed in 2010, the New START treaty caps the number of strategic nuclear warheads that Russia and the United States can deploy. Putin said in February that Russia was suspending its participation, alleging that Washington was using information obtained under the treaty to help Ukraine attack Russian strategic sites. The United States said in June it would stop providing Russia with some notifications required under the treaty, including on its missile and launcher locations, to retaliate for Moscow's "ongoing violations" of the accord.

New START is the last remaining pillar of nuclear arms control between the two countries, and is due to expire in 2026.

(Reporting by Mark Trevelyan; Editing by Gareth Jones)