Russia on Thursday charged an American correspondent for the Wall Street Journal with spying, in a case certain to escalate Moscow’s diplomatic feud with Washington over the war in Ukraine and likely to further isolate Russia.

The newspaper denied the allegations and demanded the immediate release of “trusted and dedicated reporter” Evan Gershkovich. There was no immediate response from Washington.

Gershkovich, a 31-year-old who has worked in Russia as a journalist for six years, is the highest-profile American arrested there since basketball star Brittney Griner, who was freed in December after 10 months in jail on drugs charges.

The FSB said it arrested Gershkovich in the Urals industrial city of Yekaterinburg, “suspected of spying in the interests of the American government” by collecting information on “one of the enterprises of Russia’s military-industrial complex”, which it did not identify.

He was brought to Moscow, where a court at a closed hearing ordered him held in pre-trial detention until May 29. The TASS state news agency said he pleaded not guilty. The authorities released no evidence publicly, and TASS said the case had been marked “top secret”.

Daniil Berman, a lawyer representing the reporter, was not permitted inside the courtroom or allowed to see the charges, Berman told reporters outside. He believed Gershkovich would be taken to Lefortovo, the 19th century central Moscow jail notorious in Soviet times for holding political prisoners.

“The Wall Street Journal vehemently denies the allegations from the FSB and seeks the immediate release of our trusted and dedicated reporter, Evan Gershkovich. We stand in solidarity with Evan and his family,” the newspaper said.

Kremlin spokesman Dmitry Peskov said he believed Gershkovich had been “caught red-handed”. Deputy Foreign Minister Sergei Ryabkov said it was too early to talk of any possible prisoner swap with the United States, saying that such deals are typically arranged only after a prisoner is convicted.

The US State Department’s travel guidance, last updated in February this year, advises US citizens not to go to Russia because of the danger of arbitrary arrest, and says those living or travelling there should depart immediately.

In addition to escalting Moscow’s diplomatic conflict with the United States, the case could further isolate Russia by frightening away more of the few foreign journalists still working there.

The arrest was “a frontal attack on all foreign correspondents who still work in Russia. And it means that the FSB is off the leash”, wrote Andrei Soldatov, a Russian journalist outside the country who specialises in the security services.

Moscow has effectively outlawed all independent Russian news outlets since the start of the war but has continued to accredit some foreign reporters. Journalism has become sharply limited by laws that impose long sentences for any public criticism of the war, which Russia refers to as a “special military operation”.

Kyiv acknowledges some Russian gains

Ukraine said on Thursday Russian forces had made some gains inside the eastern battlefield city of Bakhmut, but at a heavy price in lives lost that has blunted Moscow’s offensive as Kyiv prepares a counterstrike of its own.

The small mining city of Bakhmut has been the site of the bloodiest infantry battle in Europe since World War Two, with Russian forces seeking their first victory since mid-2022.

Ukraine has been on the defensive for nearly five months but says it is planning a counteroffensive soon.

“Enemy forces had a degree of success in their actions aimed at storming the city of Bakhmut,” the General Staff of the Ukrainian armed forces said in an overnight report. “Our defenders are holding the city and are repelling numerous enemy attacks.”

The report gave no details of the Russian gains. The Institute for the Study of War think tank said Russian troops and Wagner mercenaries had captured territory in the south and southwest of the city over the past two days, and Wagner had occupied a metal plant in its north this week.

Russian forces have been advancing slowly inside Bakhmut in intense street fighting. A month ago, Kyiv seemed likely to abandon the city but has since decided to stay and fight for it, hoping to break the attacking force.

Deputy defence minister Hanna Malyar said in a social media post that losses were inevitable, but “the enemy’s losses are many times greater”.

Serhiy Cherevatyi, a Ukrainian military spokesperson, told national television: “Bakhmut remains the epicenter of military activity...It’s still constantly ‘hot’ there.”

As winter has turned to spring, the pressing question is how much longer Russia can sustain its offensive, and when or if Ukraine will strike back.

Ukrainian and Western officials point to signs that Russia’s campaign is flagging. The average number of daily Russian attacks on the front line reported by Ukraine’s general staff has declined almost by half over the past four weeks.

Russia’s invasion has destroyed Ukrainian cities and set millions of refugees to flight. Tens of thousands of Ukrainian civilians and soldiers on both sides are believed to have died.

Moscow, which says it sent in troops because its neighbour posed a security threat, has vowed to press on fighting at least until it controls all the territory of eastern provinces, among five it claims to have annexed. Kyiv says it will fight on until all Russian troops are driven from its land.