U.S. President Joe Biden expressed concern over Moscow’s crackdown on Kremlin critic Aleksei Navalny and anti-government protest.

Speaking one day after mass arrests of demonstrators across Russia against President Vladimir Putin and the jailing of Navalny, Biden said on January 25 he was “very concerned.”

But he also noted that the United States and Russia will need to cooperate on nuclear arms control, with the clock ticking toward the expiration of New START, the last remaining arms control pact between Washington and Moscow, on February 5.

Biden said he considers it possible to make it clear to Russia that the United States is “very concerned about their behavior,” while both countries “operate in mutual self-interest” on the treaty.

Biden also referred to the huge breach of U.S. computer networks revealed last month, which has been blamed on Russian hackers, and reports last year that Russia offered the Taliban bounties to kill U.S. soldiers in Afghanistan.

“I have asked the agencies in question to do a thorough read for me on every one of those issues, to update me precisely where they are, and I will not hesitate to raise those issues with the Russians,” he said, speaking to reporters at the White House.

The State Department condemned “harsh tactics” used to confront nationwide demonstrations on January 23 in support of Navalny.

WATCH: Scuffles And Snowballs As Protesters Come Out For Navalny In St. Petersburg

New START limits the number of deployed strategic nuclear warheads at 1,550 and deployed strategic delivery systems at 700.

The White House said last week that Biden intends to seek a five-year extension of the deal, a proposal welcomed by Russia, but Moscow also said it needs to see details.

“Russia stands for the preservation of the New START and for its extension,” Kremlin spokesman Dmitry Peskov said on January 22. “We can only welcome the political will to extend the document, but everything will depend on the details of this proposal, which is yet to be studied.”

Navalny’s case was the subject at a meeting on January 25 of European Union foreign ministers. The bloc will wait to see if Russia releases Navalny before deciding to impose fresh sanctions.

Navalny was detained a week ago upon returning to Russia from Germany, where he had been recovering from a near-fatal poisoning by a military-grade nerve agent in August he blames President Vladimir Putin of ordering. A court is expected to decide in early February whether to imprison Navalny for a suspended sentence in a case that is widely considered trumped up and politically motivated.

Josep Borrell, the EU’s foreign policy chief, said he would go to Moscow next week to urge Moscow to free protesters and Navalny. EU leaders could discuss further action against Russia at a planned summit on March 25-26, he said.

Some EU foreign ministers are calling for tough action.

Lithuanian Foreign Minister Gabrielius Landsbergis said the 27-member bloc “needs to send a very clear and decisive message that this is not acceptable.”

Russia has rebuffed the global outrage and a chorus of international calls calling for Navalny’s release, doubling down on threats to quell any unrest.

Meanwhile, Navalny and his allies aren’t backing down, seeking to build on momentum and international focus on the issue.

Leonid Volkov, a top ally of Navalny, praised the turnout in cities and towns across all of Russia’s 11 time zones in bitterly cold temperatures as he called for fresh demonstrations on January 31.

“All cities of Russia. For freedom for Navalny. For freedom for everyone. For justice,” he wrote on Twitter.

According to the independent political watchdog OVD-Info, more than 3,700 people were taken into custody during the nationwide protests on January 23, Russia’s biggest anti-government demonstrations in years.

With reporting by AFP and Reuters