US President Trump has ordered two nuclear submarines repositioned in response to what he called “provocative” threats from former Russian President Dmitry Medvedev, according to BBC.

Trump said the move was a precaution, warning that Medvedev’s remarks could have serious consequences. The exact deployment location was not disclosed.

The tensions follow Trump’s ultimatum to Russia to agree to a Ukraine ceasefire or face sanctions. Medvedev warned such ultimatums risk war, referencing Russia’s nuclear response system.

The Kremlin has not responded, but Russian markets fell sharply after Trump’s announcement, BBC reported.