"We have made it very clear that what is happening is totally illegal," Trump said in a brief exchange with reporters before flying to Northern Ireland.

The White House later said Biden spoke by phone from the presidential plane Air Force One with Gershkovich's relatives.

The president told them he was "focused on securing Evan's release," as well as that of another U.S. citizen detained in Russia, Paul Whelan, White House spokeswoman Karine Jean-Pierre told reporters.

"We appreciate President Biden's call today, who assured us that the U.S. state is doing everything in its power to bring (Evan) home as quickly as possible," the Wall Street Journal reporter's family said in a statement released by Dow Jones, the parent company of the New York outlet.

"In addition to being a well-known journalist, Evan is a beloved son and brother. There is a void in our hearts and in our family that will not be filled until we are reunited," the journalist's relatives added.

US President Joe Biden makes a statement to the press before boarding Air Force One for Northern Ireland, on April 11, 2023 at Andrews Air Force Base, Maryland © Jim WATSON / AFP

The arrest of Evan Gershkovich was announced on 30th March by the Russian security services.

The State Department officially determined Monday that Gershkovich was "arbitrarily detained by Russia," according to U.S. Secretary of State Antony Blinken, which allows the case to be entrusted to the special envoy for hostages, Roger Carstens, and to allocate specific funds to him.

This decision by Washington "changes the dynamic" around the situation, Joe Biden said Tuesday.

Evan Gershkovich, an experienced journalist, and his newspaper "categorically" rejected the accusations of "espionage".

Russian authorities continue to deny consular access to U.S. officials, according to the White House.

Asked why the delay was delayed, John Kirby, spokesman for the White House's National Security Council, said: "This is a question to ask the Russian authorities."

"But it's not for lack of trying, I can assure you. The State Department has been trying to get consular access for (Evan Gershkovich) since we learned he was being detained," he said.

© 2023 AFP