The political operative Roger Stone has admitted he is “probably” the Donald Trump associate cited in a grand jury indictment as communicating with Russians who attacked the US presidential election in 2016.

Trump is defying calls to cancel a high stakes meeting with Vladimir Putin in Helsinki on Monday after a dozen Russian military intelligence officers were charged with hacking and leaking the emails of senior Democrats.

The indictment says the Russians, using the persona Guccifer 2.0, wrote to an unnamed person “who was in regular contact with senior members of the presidential campaign of Donald J Trump”. On 15 August 2016, according to the indictment, Guccifer 2.0 wrote: “thank u for writing back … do u find anyt(h)ing interesting in the docs i posted?”

Two days later, Guccifer 2.0 added: “please tell me if i can help u anyhow ... it would be a great pleasure to me.”

In September, the Russians wrote to the person again and referred to a stolen Democratic Congressional Campaign Committee (DCCC) document posted online: “What do u think of the info on the turnout model for the democrats entire presidential campaign.”

The person responded: “(p)retty standard”.

I am probably the person mentioned on the indictment … until there is a trial all claims remain unproven Roger Stone

The indictment does not identify the person but Stone, a political consultant and friend of Trump for nearly four decades, told the Guardian via text message: “I am probably the person mentioned on the indictment. Indictments are charges and until there is a trial all claims remain unproven.”

In a dramatic split screen moment on Friday, the charges were announced in Washington at the moment Trump was meeting the Queen at Windsor Castle. The 29-page indictment laid out how Russians schemed to break into key Democratic email accounts, including those belonging to Hillary Clinton’s campaign chairman, John Podesta, the Democratic National Committee and the DCCC. Stolen emails appeared on WikiLeaks and received widespread media coverage.

Intriguingly, the charges say that on 27 July 2016, the very day Trump held a press conference and urged Russia to find Clinton’s missing emails, Russian hackers tried for the first time to break into email accounts used by her personal office. They do not allege that Trump campaign associates were involved in the hacking effort, that Americans were knowingly in touch with Russian intelligence officers or that any vote tallies were altered by hacking.

Stone protests his innocence. He wrote on Saturday that what he described as a 24-word exchange with someone on Twitter claiming to be Guccifer 2.0 “is benign based on its content, context and timing”.

He added: “This exchange is entirely public and provides no evidence of collaboration or collusion with Guccifer 2.0 or anyone else in the alleged hacking of the DNC emails, as well as taking place many weeks after the events described in today’s indictment and after WikiLeaks had published the DNC material.”

Stone is a self-proclaimed “dirty trickster” who has a tattoo of the former president Richard Nixon on his back. He added: “The indictment does not allege or even allude that I was part of this alleged hacking nor does it allege or allude to me having anything to do with getting the allegedly hacked material to WikiLeaks. The indictments show I did not conspire with any of the defendants to do the alleged hacking, distribute the allegedly stolen emails or aid them in any way.”

‘Both pointless and dangerous’

The latest, stunning development in the special counsel Robert Mueller’s investigation into alleged collusion between the Trump campaign and Russia casts a pall over Monday’s one-on-one talks between Trump and Putin, the men who control the world’s two biggest nuclear arsenals, in a city that hosted past cold war showdowns between the superpowers.

In the US, a group of senators and another18 Democratic members of the House committee on foreign affairs called on Trump to abort the meeting. “Unfortunately,” the House group wrote, “due to your constant expressions of sympathy for Vladimir Putin, your conflicts of interest, and your attacks on our closest allies, we do not have confidence that you can faithfully negotiate with the Russian leader, and we urge you to cancel the meeting.”

The sentiment was echoed by top Democrats. Nancy Pelosi, the minority leader in the House, said: “President Trump’s continued refusal to condemn the Russians’ attacks on our democracy, even after special counsel Mueller indicted 12 Russian intelligence officials for interfering in the 2016 election, makes it clear that meeting with Putin would be both pointless and dangerous.”

Chuck Schumer, the minority leader in the Senate, said: “President Trump should cancel his meeting with Vladimir Putin until Russia takes demonstrable and transparent steps to prove that they won’t interfere in future elections.”

Trump and Putin at a 2017 summit in Vietnam. Photograph: Mikhail Klimentyev/Sputnik/Kremlin Pool/EPA

The Republican senator John McCain, a frequent Trump critic, said the summit should be called off if Trump was not ready to warn Putin there is a “serious price to pay for his ongoing aggression towards the United States and democracies around the world”.

He added: “If President Trump is not prepared to hold Putin accountable, the summit in Helsinki should not move forward.”

The White House said the summit would go ahead as planned.

If President Trump is not prepared to hold Putin accountable, the summit in Helsinki should not move forward John McCain

Trump has dismissed Mueller’s investigation as a “witch-hunt” and never condemned Putin over election interference despite the findings of US intelligence agencies. This week he said he would bring up the issue when the two meet. “He may deny it,” Trump told reporters in Brussels. “I mean, it’s one of those things. So all I can do is say, ‘Did you?’ and ‘Don’t do it again.’”

On Saturday, Trump’s first reaction to the charges was to seek to shift blame to his predecessor, Barack Obama. “The stories you heard about the 12 Russians yesterday took place during the Obama Administration, not the Trump Administration,” he tweeted from his golf resort in Scotland. “Why didn’t they do something about it, especially when it was reported that President Obama was informed by the FBI in September, before the Election?”

Russian officials largely dismissed the indictment, saying the new evidence did not prove that Russian military intelligence was behind the attack on Democratic servers or that any hack even took place. “It’s clear that the goal of this fake is to ruin the atmosphere ahead of the Russian-American summit,” the Russian foreign ministry said in a statement on Friday evening.

For Putin, the most important element of the Helsinki meeting may be in its optics, showing that the Kremlin is not isolated on the world stage despite its annexation of Crimea and alleged meddling in the US election.

Trump plays a round of golf on the Trump Turnberry resort in South Ayrshire. Photograph: Jane Barlow/PA

Dmitri Trenin, the head of the Moscow Carnegie Centre, called the meeting the “first détente in the four-year-old Hybrid War between Russia and the United States”. For Putin, he said, it could mark a resumption of normal business with the US. But it would not change the fundamental tensions in the relationship. Expectations for a grand bargain from the meeting were overblown, he said.

Syria appears to be one avenue for cooperation. At the Kremlin this week, the Israeli prime minister Benjamin Netanyahu reportedly told Putin his country would accept Bashar al-Assad’s continued rule in exchange for Moscow’s help in limiting Iranian influence in Syria.

Michael McFaul, the former US ambassador to Moscow, said in an interview a similar deal could let Trump declare victory in Helsinki and allow Russia to seek further concessions about the US role in Syria. The very image of a deal would be useful to the Kremlin, he added.

“If you are writing about a deal on Monday, that means you’re not writing about Crimea or the people who have been killed because of Putin’s decisions,” McFaul said. “He’s hoping to make us forget about those past violent actions.”

After another tumultuous week, in which he savaged Germany for its dependence on Russian energy and publicly criticised Theresa May’s Brexit strategy, there were fresh protests on Saturday as Trump and his wife, Melania, stayed at his Trump Turnberry resort in Ayrshire.

In a tweet, he gave no hint that he was wavering over whether the Putin meeting should take place.

“I have arrived in Scotland,” the president wrote, “and will be at Trump Turnberry for two days of meetings, calls and hopefully, some golf - my primary form of exercise! The weather is beautiful, and this place is incredible! Tomorrow I go to Helsinki for a Monday meeting with Vladimir Putin.”