Moscow has said that Washington is making a "serious mistake" pulling out of the treaty. (Reuters photo)

The United States vowed to upgrade its cruise and ballistic missile capability as the collapse Friday of a Cold War nuclear pact with Russia triggered fears of a new arms race.

As Washington placed the blame firmly on Moscow over the demise of the Intermediate-Range Nuclear Forces (INF) treaty, President Donald Trump said any new disarmament pact would also need China to come on board.

Russia in turn accused the US of making a "serious mistake" in turning its back on the INF, which the United Nations said had played a pivotal role in maintaining peace and stability for more than three decades.

The pact had been widely proclaimed as a beacon of hope when it was signed back in 1987 by then US president Ronald Reagan and Soviet leader Mikhail Gorbachev as they agreed to limit the use of conventional and nuclear medium-range missiles.

But its unravelling had been on the cards for months amid a worsening in ties between Russia and the US.

"Russia is solely responsible for the treaty's demise," US Secretary of State Mike Pompeo said in a statement confirming Washington's formal withdrawal, minutes after Russia also pronounced the agreement void.

Russia's suggestion of a moratorium was swiftly rebuffed by NATO chief Jens Stoltenberg as not credible while saying the 29-country transatlantic alliance did not want to see a renewed battle for military supremacy.

"We will not mirror what Russia does," Stoltenberg told a news conference in Brussels.

"We don't want a new arms race. And we have no intention to deploy new land-based nuclear missiles in Europe," he said.

But there was clear unease in European capitals, with France's foreign ministry voicing concern that the treaty's collapse "increases the risk of instability in Europe and erodes the international arms control system."

"Europe must not become the scene of a new arms race," Austrian Foreign Minister Alexander Schallenberg urged.

Under the deal, missiles with ranges of 500 to 5,500 kilometers (310 to 3,420 miles) were eliminated.

But Trump's new defense secretary said the US had already begun work to develop "mobile, conventional, ground-launched cruise and ballistic missile systems."

As the United States had "scrupulously complied" with its INF obligations while it was party to the treaty, "these programs are in the early stages," Mark Esper said.

"Now that we have withdrawn, the Department of Defense will fully pursue the development of these ground-launched conventional missiles as a prudent response to Russia's actions," he added, without saying where the missiles could be stationed.

Russia's foreign ministry said Washington was making a "serious mistake" pulling out of the treaty, adding in a statement that the US had run a "propaganda campaign" accusing Russia of violating the deal.

'Serious Mistake'

Under the deal, missiles with ranges of 500 to 5,500 kilometers (310 to 3,420 miles) were eliminated.

That paved the way for the mothballing of Russian SS-20 missiles and American Pershing missiles deployed in Europe.

For years, Washington has accused Russia of developing a new type of missile, the 9M729, which it says violates the treaty -- claims that NATO has backed up.

The missile has a range of about 1,500 kilometers, according to NATO, though Moscow says it can only travel 480 kilometers.

The White House launched a six-month withdrawal procedure for leaving the treaty in February.

Soon after, Moscow began its move to pull out, and last month Russian President Vladimir Putin formally suspended its participation.

The INF treaty was considered a cornerstone of the global arms control architecture.

But the United States accused Russia of repeated violations and said the bilateral pact had given other countries -- namely China -- free rein to develop their own long-range missiles.

Trump reiterated on Friday that he wanted to see China also sign up to any new treaty.

"We'd certainly want to include China at some point," Trump told reporters. "That would be a great thing for the world."

US-China tensions -- mainly over trade and maritime disputes -- have been center stage at the meeting of foreign ministers from the Association of Southeast Asian nations in Bangkok this week, where Pompeo has peddled Washington's "Indo-Pacific" strategy to counter Beijing's economic and military might in Asia.

Pompeo said Friday the US was "seeking a new era of arms control that moves beyond the bilateral treaties of the past" as he also called on Beijing to join discussions.

The INF deal was seen as one of two key arms deals between Russia and the US -- the other being the New START treaty, which keeps the nuclear arsenals of both countries well below their Cold War peak.

That deal expires in 2021 and there appears to be little political will from Moscow or Washington to renew it, experts say.

China has already rebuffed calls from the US to join the New START treaty in the future.