Russian Foreign Minister Sergei Lavrov dismissed concerns about Russian interference in U.S. elections and waged a bizarre personal attack on an American prisoner being held in Moscow after meeting with Secretary of State Mike Pompeo and President Trump on Tuesday.

The disagreements between Russia’s top diplomat and his American counterpart were on full display as soon as Pompeo said at a joint press conference that he’d put his foot down with Moscow and made clear the Trump administration would not put up with election meddling.

“I was clear—it’s unacceptable,” he said.

Lavrov replied that Russia had not seen any proof of election meddling, and when a reporter suggested he could simply “read the Mueller report,” he said there was “no proof of any collusion” in the report.

According to a transcript published by Russia's Foreign Ministry, he also cryptically said the Kremlin was “ready to publicize the correspondence between us and the American administration regarding allegations of interference.”

“We will be ready, as soon as Washington confirms its consent, to publicize these documents that are important to the public,” he was quoted as saying.

Reuters separately quoted him as saying: “We suggested to our colleagues that in order to dispel all suspicions that are baseless: Let us publish this close channel of correspondence starting from October 2016 until November 2017 so it would all become very clear to many people.”

It was not immediately clear what correspondence he was referring to.

Lavrov’s high-level meetings at the White House with both Pompeo and Trump sparked some criticism from those who questioned why Ukrainian President Volodymyr Zelensky was not invited instead, given that the U.S. is an ally to Ukraine in its long-running conflict with Russia. The meetings also transpired shortly after House Democrats unveiled two articles of impeachment against Trump over his pressure campaign on Ukraine to dig up dirt on his political rivals.

Trump, meanwhile, made no mention of Ukraine in a tweet summing up his “very good meeting” with Lavrov, a meeting that came just a day after peace talks in Paris between Zelensky and Russian President Vladimir Putin, along with the leaders of France and Germany.

Trump said he had also discussed “election meddling” with Lavrov, but gave no further details.

Pompeo said he’d also brought up with his Russian counterpart the issue of Paul Whelan, an American and former U.S. Marine being held in Moscow on espionage charges ever since his arrest in late 2018, just a few months after accused Russian agent Maria Butina was arrested in the U.S. The U.S. Embassy in Moscow has repeatedly raised concerns over what it described as a lack of evidence in the case, and Whelan has said through his lawyer that he was set up by a member of Russia’s intelligence service. 

Lavrov, after noting that the investigation into Whelan was already completed and the indictment filed, launched a personal attack on the 49-year-old American, claiming his lawyers should “advise him how to behave.”

Accusing Whelan of “acting defiantly” behind bars, Lavrov claimed he is aggressive toward prison guards and “threatens to bore into their heads with a drill,” according to a transcript of his comments released by Russia’s Foreign Ministry. 

Shortly after Lavrov’s allegation, which a lawyer for Whelan reportedly denied, Russia’s Foreign Ministry doubled down and released a photo on Facebook it said was of a drill similar to the kind Whelan has supposedly threatened to use on prison guards, one apparently used by investigators to bind stacks of documents together.