The Kremlin said on Thursday it regretted the expiration of the last nuclear arms agreement between Russia and the United States, while President Donald Trump said he was against keeping its limits and wanted a better deal.

The pact's termination left no caps on the two largest atomic arsenals for the first time in more than a half-century, fueling fears of an unconstrained nuclear arms race.

Russian President Vladimir Putin last year declared his readiness to stick to the treaty's limits for another year if Washington followed suit, but Trump has ignored the offer and argued that he wants China to be a part of a new pact – something Beijing has rebuffed.

"Rather than extend 'NEW START' (A badly negotiated deal by the United States that, aside from everything else, is being grossly violated), we should have our Nuclear Experts work on a new, improved, and modernized Treaty that can last long into the future," Trump posted on his Truth Social network.

Putin discussed the pact's expiration with Chinese leader Xi Jinping on Wednesday, noting the U.S. failure to respond to his proposal to extend its limits and saying that Russia "will act in a balanced and responsible manner based on thorough analysis of the security situation," Kremlin adviser Yuri Ushakov said.