A Russian firm on Thursday announced a weekly ferry service linking the Russian city of Vladivostock to North Korea's port, Rajin, despite the U.S. calling for world powers to isolate Pyongyang.

The North Korean-flagged vessel's maiden voyage Thursday was shrouded in secrecy. Reuters reported that journalists were unable to watch the vessel leave Russia due to an unknown security concern.

The ferry’s operators said that the ferry is going to be used only for commercial purposes, but experts told the news agency that North Korea could be looking for a new friend in the event that China grows cold.

Russia is one of the few countries with diplomatic ties to Pyongyang. The Washington Post reported that in 2014, Moscow wrote off 90 percent of North Korea’s $11 billion Soviet-era debt.

There are also railway deals and reportedly training opportunities for North Korean engineers.

A spokeswoman from the U.S. State Department told the news agency that the U.S. calls on all countries to “fully implement U.N. Security Council Resolutions, and sever or downgrade diplomatic and commercial relations with North Korea."

The ferry operators said they hope to attract Chinese tourists who want to visit the Russian port.

"It's our business, of our company, without any state subsidies, involvement and help," Mikhail Khmel, an official from Investstroytrest, the firm operating the ferry, told the news agency.

Russian President Vladimir Putin has in the past spoken out against Pyongyang’s missile launches, but he also warned other countries not to intimidate Pyongyang.

“I would like to confirm that we are categorically against the expansion of the club of nuclear states, including through the Korean Peninsula,” Putin said, earlier this month. “We are against it and consider it counterproductive, damaging, dangerous.”