Russia charged an American journalist with spying while NATO moved closer to adding Finland as a member, deepening Moscow's tensions with the West as the war in Ukraine reached its 400th day on Friday.

Ukrainian President Volodymyr Zelenskiy spoke in his Thursday evening video address of the "tremendous path" his country had taken in 400 days of resistance since Russia's invasion on Feb. 24, 2022.

"Ukraine will win at the front ... we will not leave a single trace of Russia on our land, and we will not leave any enemy unpunished either. We are preparing news about this," he said. He did not give details, but the Ukrainian military has been planning a counteroffensive.

Earlier on Thursday, the Wall Street Journal denied espionage allegations against its reporter Evan Gershkovich and demanded his immediate release. The Kremlin said he had been "caught red-handed" but presented no documentary or video evidence.

The White House condemned Russia's actions and urged U.S. citizens living or traveling in Russia to leave immediately.

"These espionage charges are ridiculous. The targeting of American citizens by the Russian government is unacceptable," White House Press Secretary Karine Jean-Pierre said at a news briefing.

The Russian FSB security service said it arrested Gershkovich, a 31-year-old who has worked in Russia as a journalist for six years, in the Urals industrial city of Yekaterinburg, "suspected of spying in the interests of the American government."

He was brought to Moscow, where a court at a closed hearing ordered him held in pre-trial detention until May 29. Espionage under Russian law can be punishable by up to 20 years in jail.

Western political analysts speculated Moscow could seek to barter Gershkovich in a prisoner swap with Washington or use him as diplomatic leverage in some other way.

Separately, Turkey's parliament approved a bill on Thursday to allow Finland to join NATO. The Turkish parliament was the last among the 30 members of the Western defence alliance to ratify Finland's membership after Hungary's legislature approved a similar bill earlier this week. Sweden has also sought NATO membership.

NATO countries individually, notably the United States, UK and European nations, and the alliance collectively have provided major military and financial support to the Kyiv government.

"Soon both Finland and Sweden will be (NATO) members, meaning that President Putin is getting the exact opposite of what he wanted," NATO Secretary General Jens Stoltenberg told Fox News. "He wanted less NATO. He is getting more NATO."

Russia has repeatedly accused the West and NATO of playing a direct role in the Ukraine conflict by supplying weapons to Kyiv and has warned that NATO weapons are "legitimate targets" for its armed forces.

Ukraine said on Thursday that Russian forces continued their assault on the eastern battlefield city of Bakhmut and nearby towns as well as on the contested city of Avdiivka and surrounding area.

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.

"Our defences are holding the city and repelling numerous enemy attacks," the General Staff of the Ukrainian Armed Forces said in a report on Facebook on Thursday evening. Reuters could not independently confirm the battlefield reports.

The Ukrainian military had said a day earlier that Russian fighters were having some success in Bakhmut. Evhen Dikyi, a Ukrainian military analyst interviewed on Ukrainian NV Radio on Thursday, said Russian forces control more than half the city.

"I can tell you that Bakhmut is holding firm. But there is heavy fighting in the city and it is getting closer to the city center," he said.

"If Russian forces are reported to have made 'some progress' it is probably because they have crossed the Bakhmutka River. They have come wave upon wave and we are talking about a few hundred meters," Dikyi added.

A month ago, the Ukrainian military seemed likely to abandon Bakhmut but has since decided to stay and fight for it, hoping to break the attacking force.

Serhiy Zgurets, director of the Ukrainian defence publication Defense Express, wrote in a column on the website of Ukrainian Espreso TV on Thursday that Zelenskiy was determined to defend Bakhmut because its loss would give Russia leverage to force Ukrainian concessions.

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 neighbor posed a security threat, has vowed to press on fighting at least until it controls all the eastern provinces, among five it claims to have annexed.

(Reuters)

Reporter for U.S. newspaper The Wall Street Journal Evan Gershkovich, detained on suspicion of espionage, leaves a court building in Moscow, Russia March 30, 2023. (REUTERS/Evgenia Novozhenina)

Reporter for U.S. newspaper The Wall Street Journal Evan Gershkovich appears in an undated handout image taken in an unknown location. (The Wall Street Journal/Handout via REUTERS)

Russia's isolation over Ukraine grows with US reporter arrest, NATO expansion