Washington (CNN) A top Democratic senator on Wednesday urged the Trump administration not to lift sanctions on two Russian firms, including the world's second-largest aluminum maker, until Russian billionaire Oleg Deripaska fully divests his stake in both companies.

"I see no reason to remove sanctions against Mr. Deripaska, and until he divests from and relinquishes control of RUSAL and EN+, there is no justification to remove the sanctions on those companies," New Jersey Sen. Bob Menendez, ranking member of the Senate Foreign Relations Committee, wrote in a letter to Treasury Secretary Steven Mnuchin

In April, the Trump administration announced it would impose sanctions on Deripaska , who has close ties to Russian President Vladimir Putin and his business empire, to prevent him and his companies from using the dollar and doing business with US citizens.

The New Jersey senator warned that the Trump administration would encounter "strong opposition in Congress" to any sanctions relief for Rusal and EN+, the company that holds the oligarch's stake in the Russian aluminum giant, and GAZ Group, a Russian automotive conglomerate, unless they sever ties with Deripaska and his family members as well as meeting other criteria.

"How the Treasury Department manages this delisting exercise will shape our perceptions about the administration's seriousness in implementing the Russia sanctions regime," Menendez wrote.

A Treasury spokesman did not return a request for comment by CNN.

Deripaska was sanctioned earlier this year as part of a wider effort by the Trump administration to punish the Kremlin for its "malign activity" around the world, including interference in the 2016 US presidential election.

Deripaska, who also had a longstanding business relationship with former Trump campaign chairman Paul Manafort, faces claims from Treasury of bribery, extortion, racketeering, ordering the murder of a rival businessman and having links to organized crime. In statements provided to London High Court, Deripaska has denied the "alleged basis" of US sanctions

Sanctions were supposed to take effect immediately, but Rusal quickly asked the Treasury Department to lift them. The US government agreed to consider it and gave Rusal until October 23 to unwind its ties to Deripaska. Since then, the Treasury Department has extended the deadline multiple times to allow Deripaska additional time to give up control of his stakes in both firms.

Treasury has been trying to balance its effort to squeeze Russia with its desire not to tank the markets. In November, Treasury said licenses allowing Rusal to continue to do business with existing supply deals would be extended to January 7, 2019. The previous deadline was December 12.

Enforcement of the economic penalties against Russia, including Rusal, have been a priority for the Trump administration and are required under a law passed by Congress last summer to punish the Kremlin.

The law passed last year gives Treasury new powers to go after Russian oligarchs. Trump reluctantly signed the bill, which was passed with a veto-proof majority, despite criticizing the legislation and previously questioning the effectiveness of sanctions against Russia.

Mnuchin has committed to enforcing the Russia sanctions, but has made a point of saying the intent is not to put Rusal out of business. "The objective was to impact oligarchs, not to impact the hardworking people of Rusal as a result of sanctions," Mnuchin told CNN in an interview in July