Russia on Saturday said it would expel a Ukrainian diplomat it accused of trying to obtain classified information, prompting Ukraine to say it would do the same to a Russian diplomat, according to Reuters.

Why it matters: The tit-for-tat expulsions come amid heightened tensions between the two countries over Russia's build-up of military personnel and equipment on Ukraine's eastern border where Ukrainian soldiers are fighting Kremlin-back separatist troops.

Jailed Russian opposition leader Alexei Navalny faces possible kidney failure as he continues his multi-week hunger strike, a medical trade union told Reuters on Saturday.

What they're saying: "His condition is indeed critical ... His potassium is high and he has other high readings which indicate that his kidneys may soon fail. This would lead to severe pathology and cardiac arrest may occur," Alexandra Zakharova, a representative for the Doctors Alliance trade union, told Reuters.

Russia will expel 10 U.S. diplomats and add eight current and former U.S. officials to its no-entry list in retaliation for sanctions that the Biden administration leveled at Moscow on Friday, Russian foreign minister Sergey Lavrov said Friday.

Why it matters: The measures come after the U.S. said it would expel 10 Russian intelligence officers operating under diplomatic cover, as part of a broad package of sanctions retaliating against the SolarWinds hack of federal agencies and Russia's interference in the 2020 election.

Despite bold talk from top administration officials, there's little reason to think the Russia sanctions package President Biden announced Thursday will do anything to alter Russian President Vladimir Putin's behavior or calculus.

Why it matters: While it's true some elements of the package — namely, the targeting of Russia's sovereign debt — represent significant punitive measures against Moscow, it leaves plenty of wiggle room for the Russian president.

U.S. intelligence officials have "low to moderate confidence" in reports that surfaced last year that Russia had offered bounties to Taliban-linked militants for killing U.S. troops stationed in Afghanistan, according to The Guardian.

Why it matters: The news comes as the Biden administration unveiled a spate of sanctions against Russian officials and entities on Thursday. The bounty reports, however, were not a factor in the decision to pass sanctions.

The Biden administration announced it will sanction dozens of Russian officials and entities, expel 10 diplomats from the U.S., and set new restrictions on buying Russian sovereign debt in response to the massive SolarWinds hack of federal agencies and interference in the 2020 election.

Why it matters: The sweeping acts of retaliation are aimed at imposing heavy economic costs on Russia, after years of sanctions that have failed to deter an increasingly aggressive and authoritarian President Vladimir Putin.

The U.S. government has sanctioned Konstantin Kilimnik, a Russian-Ukrainian political consultant indicted in the Mueller investigation in 2018, for carrying out election influence operations on behalf of Russian intelligence services.

The big picture: The Senate Intelligence Committee's report on 2016 Russian election interference assessed that Kilimnik, who worked with former Trump campaign manager Paul Manafort as a lobbyist for the pro-Russia president of Ukraine, is a Russian intelligence officer.

President Biden spoke with Russian President Vladimir Putin on Tuesday and proposed that they meet for a summit "in a third country in the coming months," according to the White House.

Why it matters: The call comes amid a Russian build-up on Ukraine's borders, and after Putin reacted furiously to an interview in which Biden agreed that the Russian president was a "killer."

President Biden is considering appointing Matthew Rojansky, head of the Wilson Center's Kennan Institute, as Russia director on the National Security Council, according to a source familiar with the situation.

Why it matters: Rojansky has been praised for his scholarship on Russia and is frequently cited in U.S. media for his expert commentary. But his work has drawn criticism — including in a 2018 open letter from Ukrainian alumni of Kennan that blasted the think tank he runs as an "unwitting tool of Russia’s political interference."

The simmering conflict in Eastern Ukraine has threatened to boil over the past few days, with Russia massing troops near the border and pro-Kremlin media raising the specter of war.

Why it matters: U.S. European Command went into high alert in light of the Russian movements, which some experts speculated could presage an active Russian military intervention in the Donbas region of Eastern Ukraine, where Ukraine has been fighting pro-Russian separatists for seven years.