Russia's President Vladimir Putin has said his country will abandon a landmark nuclear arms treaty, following in the footsteps of the United States, and that Moscow will only deploy intermediate-range nuclear missiles if Washington does so.
The move by both leaders has caused concern among analysts who worry it could spark a new arms race.
Mr Putin spoke after Donald Trump announced on Friday that he was pulling the plug on the 1987 Intermediate-Range Nuclear Forces Treaty (INF) over alleged Russian violations.
Moscow has denied any breaches, arguing that a missile it unveiled in January meets the specifications of the treaty, and accused Washington of making false accusations in order to justify its retreat.
- Political Reporter David Wood has the latest on the treaty, and why China may be an influencing factor.
- What is the INF Treaty?
The INF Treaty was the first arms control measure to ban an entire class of weapons: ground-launched cruise missiles with a range between 310-3,400 miles (500-5,500km).
The deal was signed by the US and the USSR in 1987, and by 1991, nearly 2,700 missiles had been destroyed.
The treaty allowed both countries to inspect the other's installations.
In 2007, President Putin declared the treaty no longer served Russia's interests, a move which came after the US withdrew from the Anti-Ballistic Missile Treaty in 2002.
- Why has the US pulled out of the treaty?
The US has accused Russia of violating the pact with "impunity", developing and deploying a ground-based cruise missile that violates provisions of the pact that ban production, testing and deployment of land-based cruise and ballistic missiles with a range of 310-3,400 miles.
Moscow denies that it has done this.
US officials have also expressed worry that China - which is not party to the 1987 treaty - is gaining a significant military advantage in Asia by deploying large numbers of missiles with ranges beyond the treaty's limit.
Leaving the INF treaty would allow the Trump administration to counter the Chinese, but is currently unclear how this could be achieved.
- Has Russia breached the treaty?
Moscow has strongly denied any breaches and accused Washington of making false accusations in order to justify its pullout.
Russia instead argued that the missile which it has developed is part of the Iskander-M missile system and has a maximum range of 298 miles (480km), meaning it is not covered by the treaty.
President Putin has also argued that it makes no sense for Russia to deploy a ground-based cruise missile violating the treaty because it has such weapons on ships and aircraft, which are not banned by the pact.
The treaty only covers ground-based missiles.
However, NATO allies have strongly backed Washington and urged Moscow to save the treaty by returning to compliance.
On Saturday, the Russian Defense Ministry released a satellite image of what it described as new production facilities at the US missile maker Raytheon's plant in Tucson, Arizona, noting that their expansion began in 2017 as the Congress authorised spending for the development of intermediate-range missiles.
"The character and the timing of the works provide an irrefutable proof that the US administration had decided to pull out of the INF treaty years before making unfounded claims of Russian violations," it said.
- What else has Russia said?
After the US announced on Friday that it will withdraw from the treaty, Russia announced the same on Saturday.
"We will respond quid pro quo," President Putin said.
"Our American partners have announced they were suspending their participation in the treaty and we will do the same.
"They have announced they will conduct research and development, and we will act accordingly."
Russia's Foreign Minister added that the US withdrawal from the pivotal pact has further upset strategic stability.
Sergey Lavrov said that Washington has ignored Russia's offer to inspect a cruise missile that the US claimed violated the pact.
He claimed that the US itself had violated the treaty by deploying missile interceptors in Romania that use the launchers that could hold land-based cruise missiles.
President Putin has also ordered the development of new land-based intermediate-range weapons but emphasised that Russia will not deploy them unless the US does.
- What happens now?
Withdrawal from the pact will take effect six months after the notification, meaning a small window remains for saving the treaty.
However, in talks this week in Beijing, the US and Russia reported no breakthrough in their dispute, leaving little hope for future discussions.
Mr Putin has said that Moscow remains open to talks with Washington, but added it would be up to the US to take the first step.
"Let's wait until our partners are mature enough to conduct an equal and substantive dialogue on those issues," he said.
Mr Lavrov also said that another centrepiece nuclear arms pact, the New Start, which is set to expire in 2021, is also in trouble.
- Are we headed for a nuclear war?
The collapse of the treaty has raised fears of a repeat of a Cold War showdown in the 1980s, when the US and the Soviet Union both deployed intermediate-range missiles on the continent.
Such weapons were seen as particularly destabilising as they only take a few minutes to reach their targets, leaving no time for decision-makers and raising the likelihood of a global nuclear conflict over a false launch warning.
However, due to the destructive power of nuclear weapons, it is unlikely that the US and Russia will launch them at each other.