WASHINGTON (WASHINGTON POST) - The United States military intelligence agency stepped up its accusations against Russia over low-yield nuclear testing on Thursday (June 13), saying that the country has conducted nuclear weapons tests that resulted in nuclear yield.

The new statement from the Defence Intelligence Agency (DIA) amounted to a more direct accusation against Russia, compared to hedged comments about Russian nuclear testing that agency director Lieutenant-General Robert Ashley Jr made in a speech in Washington in late May.

"The US government, including the Intelligence Community, has assessed that Russia has conducted nuclear weapons tests that have created nuclear yield," the DIA statement released on Thursday said. The agency did not give any details about the alleged tests or release any evidence backing the accusation.

The US Intelligence Community is a federation of 17 government intelligence agencies that work separately and together to conduct intelligence activities in support of the country's foreign policy and national security.

Previously, the agency's director said that Russia probably was not adhering to the zero-yield standard the US applies for nuclear testing. He suggested that Russia was probably conducting tests with explosions above a subcritical yield as part of its development of a suite of more-sophisticated nuclear weapons.

Russia has vehemently rejected Washington's accusations, with Russian Foreign Minister Sergei Lavrov describing them as delusional.

"We consider claims that Russia may be conducting very low-yield nuclear tests as a crude provocation," Russia's Foreign Ministry said in a statement after the DIA first aired the allegations. "This accusation is absolutely groundless and is no more than another attempt to smear Russia's image."

DIA's latest accusation came a day after Under Secretary of State for Arms Control and International Security Andrea Thompson met Russian Deputy Foreign Minister Sergei Ryabkov in Prague to discuss arms control.

The meeting did not result in any significant decisions. After the meeting, Ms Thompson said in a message on Twitter that she raised a range of issues on which the US would like to engage in a more constructive dialogue with Russia.

During the meeting, Mr Ryabkov said he reiterated the unacceptability of Washington's accusations regarding alleged Russian nuclear testing and the Comprehensive Nuclear-Test-Ban Treaty, or the CTBT, a 1996 agreement that prohibits nuclear explosions.

"We said that we are in full and absolute compliance with the agreement, ratified by Moscow, and in full compliance with our unilateral moratorium on nuclear testing," Mr Ryabkov said, according to the Russian state news agency RIA Novosti.

The DIA's original accusations in late May came after the Wall Street Journal reported that a new US intelligence assessment found Russia likely has tested nuclear weapons with very low yields at Novaya Zemlya, a collection of islands in the Arctic Circle.

It was not clear how that intelligence assessment - which deemed the Russian tests likely - squared with the DIA's statement on Thursday, which stated flat out that Russia had conducted such tests. It also was not clear whether the DIA was making its statements based on new intelligence or a reinterpretation of past information.

In addition to provoking a rebuke from Russia, the US government's accusations about Russian low-yield testing have prompted scepticism among some disarmament advocates.

The James Martin Centre for Nonproliferation Studies in Monterey, California, said its regular open-source monitoring of Novaya Zemlya had not detected any alarming activity.

In a statement late last month, the centre's Eurasia programme director Sarah Bidgood said that with no new evidence that Russia is conducting low-yield tests, it appears that US officials are rehashing past allegations about Russian test violations "in order to support the narrative that Russia is an unreliable partner in arms control, with whom verification does not work".

Ms Bidgood said the accusations could be an effort by US officials to lay the groundwork for a decision to not pursue an extension of the 2010 New Start accord with Russia, which expires in early 2021.

The accusations by the DIA come as top Republicans criticise the CTBT. In a March letter to the White House, Republican Senators Tom Cotton, Marco Rubio, John Cornyn and James Lankford asked President Donald Trump whether he would consider unsigning the CTBT and called the pact a "deeply flawed treaty that purports to ban all nuclear weapons tests".

The senators expressed concern that efforts around the world to bring the CTBT into force rest on "false premises" about what was agreed to, which in turn have "given rise to wishful thinking about Russian and Chinese compliance with the treaty".

Both Russia and the US have signed the CTBT, but the pact has not come into force, because a number of countries, including the US, have not ratified it. Russia ratified the treaty in 2000. The US Senate did not consent to its ratification.

What exact standard the CTBT sets for nuclear testing is a point of contention. US opponents of the treaty point out that the pact does not define a nuclear test. They say that while the US adheres to a strict zero-yield interpretation, other countries could claim to be abiding by the treaty but still be conducting tests that Washington would consider off-limits.

A White House memo from 1997, reviewed by The Washington Post, said that the US decided at the outset of the CTBT negotiations that it was "unnecessary, and probably problematic, to seek to include a definition in the Treaty text of a 'nuclear weapon test explosion or any other nuclear explosion' for the purposes of specifying in technical terms what is prohibited by the Treaty".

Instead, according to the memo, the US, Britain, France, China and Russia exchanged letters on Sept 23, 1996, confirming an oral understanding they reached earlier that year about what activities would still be allowed under the CTBT.

The list was illustrative but not exhaustive, raising the possibility that other activities could be permitted under certain interpretations.

A senior Trump administration official said that as a result, the CTBT does not expressly stipulate a zero-yield standard because of the lack of clarity, opening the door for Russia and China to conduct tests, while the US, Britain and France adhere to a stricter interpretation.

"If we're going to continue to hold ourselves to a zero-yield standard, and they're not, that's a problem," the official said.

Mr Daryl Kimball, executive director of the Arms Control Association, disagreed that the US and Russia are operating under different interpretations of what type of nuclear test explosions are banned by the treaty.

"Both countries agree that all nuclear test explosions are banned by the treaty," Mr Kimball said. He said the US, if it has real concerns about secretive Russian testing, should propose mutual confidence-building visits allowed for under the treaty. He said the articles of the treaty pertaining to those visits can be invoked even though the agreement has not come into force.

A spokesman for the Russian Embassy in Washington pointed out that Russia stated its interpretation of nuclear testing in a 2017 article co-written by Mr Ryabkov. The article, published jointly in the Russian newspaper Kommersant and the American magazine the Diplomat and posted on the Russian Foreign Ministry's website, notes that the CTBT prohibits any nuclear weapon test explosion "anywhere on Earth, whatever the yield".