U.S. President Joe Biden's national security adviser, Jake Sullivan, said that the United States is eyeing more sanctions against Russia for the poisoning of the country's most prominent opposition leader, Alexei Navalny.

Sullivan said that they are preparing another package of sanctions over the matter. However, he did not say the timing of the sanctions or what is included in the said package.

Sullivan said that the action would come as soon as they develop the sanctions to ensure that they have the right targets, according to The New York Times report.

The national security adviser noted that they will impose further sanctions concerning chemical weapons.

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Russia's Opposition Alexei Navalny

Navalny is currently imprisoned and was reportedly in poor health. In April, Russia's penitentiary service said that it was transferring Navalny to a prison hospital, according to a New York Post report.

Navalny was said to be on a hunger strike in April.

The decision to transfer Navalny came after the U.S. threatened the Russian government with consequences if Russian President Vladimir Putin's opposition dies behind prison bars.

Navalny's private doctors earlier said that he could die at "any minute."

Russia maintained that Navalny's condition was satisfactory, saying that he was taking vitamin supplements as part of his medical treatment.

Navalny was arrested in January after returning to Russia from Germany. He was recuperating at the time from being poisoned on two separate occasions with a Soviet-era nerve agent, according to another NY Post report.

The critic blamed the attempts on his life on the Kremlin. However, Russian officials have continued to deny the accusations.

Meanwhile, Sullivan said that they rallied with European allies in a joint effort to impose sanctions on Russia for the use of a chemical agent against one of their citizens, according to a South China Morning Post report.

Moscow warned of a Russian response to the said sanctions.

Foreign Ministry spokeswoman Maria Zakharova said that the illegal actions of the U.S. have always been followed by a legitimate response from them.

Anatoly Antonov, the Russian ambassador to the U.S., said that this was not the signal that everyone got after the summit.

Putin and Biden Meeting During The Summit

Biden has warned Putin over ransomware during the summit in Geneva. The two leaders had agreed to start consultations over the issue.

Task experts from the two countries will be asked to work on specific understandings of what are off-limits for cyberattacks, according to a Voice of America report.

The U.S. president said he had given Putin a list of 16 key sectors, including energy and water services that he insisted out of bounds.

The list had also included information technology, health care, public health, and food and agriculture. These have been the targets of cyberattacks in recent years.

Biden and Putin did not note whether they talked about the criminal exchange idea. It also remains unclear how the two countries will cooperate over cybersecurity attacks coming from both countries, but not directly involving the government.

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WATCH: US warns of consequences if jailed Kremlin critic Navalny dies - from Al Jazeera English