Russia has withdrawn from a treaty that bans the use of short- and medium-range nuclear missiles, a day after the US pulled out. President Vladimir Putin said Russia will develop new high-tech missiles, potentially signaling the start of a new technological arms race.

Background: The US withdrew from the Intermediate-Range Nuclear Forces Treaty, signed in 1987, on Friday. It said Russia was violating the treaty’s terms and had six months to become compliant.

What’s the complaint? The US and NATO are unhappy about the Novator 9M729, a land-based cruise missile with a range larger than the 500-kilometer limit set by the treaty. Meanwhile, Russia is unhappy with the launchers that form part of a missile defense system used by the US, and which are stationed in Romania. Russia says they could also be used for attacks.

A new weapon: The breakdown of the treaty could herald a worrying new era in weapons development. In a Kremlin transcript of a meeting with his foreign minister and defense minister (above), Putin said that Russia’s response would be “symmetrical.” But he also said work should begin on a new project to develop a hypersonic land-based intermediate-range missile. Hypersonic means it travels at at least five times the speed of sound.

Putin boasted about Russia’s ongoing work on hypersonic missiles last year, saying they would strike “like a fireball.” The first tests of a hypersonic system called Avangard were carried out last year, he announced in December. The US is also working on hypersonic missiles, as is China. There is currently no way to defend against such weapons.