Russian media reporting suggests authorities are preparing to start a major military recruitment aiming to sign up an additional 400,000 troops, the United Kingdom's Defense Ministry assessed.

Russia is presenting the campaign as a drive for volunteer, professional personnel, rather than a new, mandatory mobilization. There is a realistic possibility that in practice this distinction will be blurred, and that regional authorities will try to meet their allocated recruitment targets by coercing men to join up, UK officials said.

Russian authorities have likely selected a supposedly ‘volunteer model’ to meet their personnel shortfall in order to minimize domestic dissent. It is highly unlikely that the campaign will attract 400,000 genuine volunteers, according to UK officials.

However, rebuilding Russia’s combat power in Ukraine will require more than just personnel; Russia needs more munitions and military equipment supplies than it currently has available, UK officials said.

The Wall Street Journal said on Thursday that it "vehemently denies" the spying allegations brought by Russia's intelligence service against its reporter.

"The Wall Street Journal vehemently denies the allegations from the FSB and seeks the immediate release of our trusted and dedicated reporter, Evan Gershkovich," a spokesperson for the WSJ said in a statement to ABC News. "We stand in solidarity with Evan and his family."

The FSB said it detained Gershkovich in the city of Ekaterinburg, in central Russia, and accused him of collecting "state secrets" on an enterprise belonging to Russia's military industrial complex on behalf of the United States.

Russia's FSB intelligence agency said on Thursday it had detained a journalist working for The Wall Street Journal on spying charges.

Russian state media cited an FSB statement saying Evan Gershkovich was detained in Ekaterinburg, a city in central Russia, and accusing him of collecting "state secrets" on an enterprise belonging to Russia's military industrial complex on behalf of the United States.

A criminal case has been opened against him, the officials said.

"It is established that Evan Gershkovich, acting on the instruction of the American side, was collecting information consisting of state secrets, about the activity of one of the enterprises of the Russian military industrial complex. He was arrested in Ekaterinburg during an attempt to receive secret information," Russian media said, quoting FSB officials.

Earlier reports from local media said that Gershkovich had been in Ekaterinburg reporting on the Wagner private military company.

Gershkovich is a reporter for the WSJ covering Russia, Ukraine and the former Soviet Union. He previously reported for Agence France-Presse and The Moscow Times, according to his WSJ profile. He also served as a news assistant at The New York Times.

-ABC News Tanya Stukalova and Patrick Reevell

The U.S. will support the creation of a special tribunal to prosecute top Kremlin officials for Russia's aggression toward Ukraine, State Department officials said Tuesday, marking a significant shift for the Biden administration and a notable step toward outlining what accountability on the international stage might look like after the conflict.

A department spokesperson said the administration envisioned the tribunal would take the form of an international court that is "rooted in Ukraine's judicial system" but ideally located in another European country.

The spokesperson added that such a mechanism would work to "facilitate broader international support and demonstrate Ukraine's leadership in ensuring accountability for the crime of aggression" as well as "maximize the chances of achieving meaningful accountability for the crime of aggression."

Ukraine and other Western countries have long called for a special tribunal, but until now, the U.S. has not publicly declared if it would support the creation of a new structure.

-ABC News' Shannon K. Crawford

Ukrainian Defense Minister Oleksii Reznikov said Friday the military is "ready" to launch a counteroffensive against Russian forces.

"It's up to the general staff and the command," Reznikov said during a press briefing in Kyiv. "We will do it as soon as there is God's will, the weather and the commanders' decision."

Ukraine has received Patriot missile defense systems from the United States as well as Germany and the Netherlands. The Ukrainian military has been trained on how to use the systems and "mastered" them within weeks, according to Reznikov.

"The exact number of batteries, I'm sorry, I won't say," he added. "Let the enemy guess."

Meanwhile, Ukrainian Foreign Minister Dmytro Kuleba urged the world not to consider or call the anticipated counteroffensive "a decisive battle." Speaking at a press conference in Odesa on Friday, Kuleba said the decisive battle is the one that will lead to the liberation of all occupied Ukrainian territories.

-ABC News’ Will Gretsky