WASHINGTON, June 20 (DW) - The United States is readying a new package of sanctions against Russia over last year's poisoning of opposition leader Alexei Navalny, a top official said on Sunday.

US President Joe Biden's national security adviser, Jake Sullivan, issued the warning during an interview with CNN.

"We have sanctioned Russia for the poisoning of Alexei Navalny ... We rallied European allies in a joint effort to impose costs on Russia for the use of a chemical agent against one of their citizens on Russian soil," Sullivan said.

"We are preparing another package of sanctions to apply in this case, as well. We've shown along the way we're not going to pull our punches," Sullivan added.

The US and the European Union already imposed sanctions over the poisoning back in March. That round of measures targeted the head of Russia's FSB security agency, which is believed to have been behind the poisoning.

Navalny, one of Putin's most prominent opponents, fell ill on August 20 last year during a domestic flight in Siberia. Two days later, he was flown to Berlin for treatment while still in a coma.

Experts determined he'd been poisoned with a Novichok nerve agent. Labs in Germany, France and Sweden also confirmed the Soviet-era agent.

After recovering, he returned to Russia from Germany in January this year — and was detained directly upon landing.

Navalny was sentenced to over two years in a penal colony in February on old embezzlement charges. His allies say his incarceration is an attempt to silence political opposition to Putin.