WASHINGTON, March 15, 2024 (BSS/AFP) - The White House said Thursday it was very concerned by a "terrible, brutal" hammer attack in Lithuania on Leonid Volkov, an ally of late Russian opposition leader Alexei Navalny.

National Security Council spokesman John Kirby said he could not confirm Lithuanian claims that Russia's special services were involved, but said the assault was a reminder of the "very real threats" to Kremlin opponents.

"We are very, very concerned about this attack on Leonid Volkov," Kirby told reporters in a call.

He said there were "just terrible, terrible, brutal reports coming out about the attack on him by an individual wielding just incredible violence and we certainly wish him the best for a speedy recovery."

The spokesman said that United States officials "don't have much more detail on who did it and the motivation behind it" at this stage.

But Kirby said the attack "should serve to all of us as a reminder of the very real threats that civil society members in Russia face on a daily basis."

"It certainly underscores the danger and the perils that they face just for the simple act of speaking out in opposition," Kirby added.

Volkov was briefly admitted to hospital after he was repeatedly struck with a hammer outside his home in Lithuania's capital Vilnius late on Tuesday.

Lithuania's intelligence chief said Thursday it "seems to be the work of the Russian special services, apparently done through some recruited person."

The attack came almost a month after Navalny's death in an Arctic prison, which Volkov blamed on Putin, and days before elections that look set to extend the Kremlin chief's stay in power.

The Kremlin said it could not comment on events in Lithuania but added that Putin "should not be feared, but respected and listened to."