"His arrest is of great concern. It is important to respect freedom of the press, the rights of journalists and the right to ask questions and to do their job," he said.

Stoltenberg said he expected the issue of the Wall Street Journal reporter's detention to be taken up by NATO foreign ministers at their meeting on Tuesday.

US Secretary of State Antony Blinken will be in Brussels to meet his 29 NATO counterparts and to welcome Finland as the 31st member of the alliance.

Gershkovich, a 31-year-old US citizen, was detained by Russian security agents last week and "is believed to be the first foreign journalist held for spying in post-Soviet Russia".

His employer has dismissed the claim that he was engaged in espionage, and Blinken has already demanded his release in a call to Russian foreign minister Sergei Lavrov.