US President-elect Donald Trump released a bromance letter he received from Russian Prime Minister Vladimir Putin, who pledged to improve relations between the two superpowers.

“Please accept my warmest Christmas and New Year greetings,” the Russian strongman, who was accused of aiding Trump in the presidential election by hacking Hillary Clinton and DNC private emails, said.

“I hope that after you assume the position of the President of the United States of America we will be able — by acting in a constructive and pragmatic manner — to take real steps to restore the framework of bilateral co-operation in different areas as well as bring our level of collaboration on the international scene to a qualitatively new level.”

During a press conference in Moscow, Putin also called Hillary Clinton and Democrats sore losers, and insisted Russia did not interfere in the presidential election and praised Trump for understanding “the mood of the people.”

Meantime, Trump upped the stakes on Friday in a back-and-forth exchange with President Vladimir Putin over nuclear weapons that tested the Republican’s promises to improve relations with Russia.

Offering a glimpse of how he might conduct diplomacy after taking office on January 20, Trump reportedly welcomed a nuclear arms race with Russia and China and boasted that the United States would win it.

MSNBC host Mika Brzezinski said Trump told her in an off-air phone call: “Let it be an arms race. We will outmatch them at every pass and outlast them all.” The television station did not play his comments on air.

It was the second brusque comment about atomic weapons in two days from the New York businessman that alarmed nuclear non-proliferation experts worried about fuelling global tension.

The broadsides from Trump’s resort in Florida appeared to be aimed mostly at Putin even though the two men have vowed to patch up relations between their countries once the Republican enters the White House.

Trump tweeted unexpectedly on Thursday that, “The United States must greatly strengthen and expand its nuclear capability until such time as the world comes to its senses regarding nukes”, but gave no further details. That comment appeared to be a response to Putin who said earlier on Thursday that Russia needed to “strengthen the military potential of strategic nuclear forces.”

Russia and the United States are at odds over Syria’s civil war and Ukraine but Cold War-style nuclear tensions have greatly eased in recent years. Moscow and Washington signed the New START nuclear treaty in 2010 which reduced strategic weapons and delivery systems.

Putin, accused by the Obama administration of overseeing a wave of cyber attacks against US political organisations during the presidential campaign, said on Friday he had no interest in competing with the US nuclear weapon program. “If anyone is unleashing an arms race it’s not us ... We will never spend resources on an arms race that we can’t afford,” he said at a news conference. The Russian president said he was surprised by State Department comments that the US military is the most powerful in the world. “Nobody is arguing with that,” Putin said.

He said he did not regard the United States as a potential aggressor, and said he saw nothing new or remarkable about Trump’s own statement about wanting to expand US nuclear capabilities.

The United States is in the midst of a $US1 trillion, 30-year modernisation of its ageing nuclear arsenal and replacement of its ballistic missile submarines, bombers and land-based missiles. It is a price tag that most experts say the United States can ill afford.

Russia, also bound by New START limits, is carrying out its own costly modernisation program but is not expanding its warhead stockpile. Trump spokesman Sean Spicer said the president-elect’s nuclear comments were meant to send a general message of strength to countries like Russia and China rather than indicate the United States planned to build up its nuclear capabilities.

Trump was elected president unexpectedly last month partly on a platform of building up the US military but he also pledged to cut taxes and control federal spending.

— with the New York Post