The U.S. believes Iranians are "on the ground" in Ukraine to assist Russia with its drone operations, White House spokesman John Kirby told reporters Thursday.

"We can confirm that Russian military personnel that are based in Crimea have been piloting Iranian [drones] and using them to conduct strikes across Ukraine, including strikes against Kiev in just recent days. We assess that Iranian military personnel were on the ground in Crimea and assisted Russia in these operations," Kirby told reporters.

Kirby did not know how many Iranians are in Crimea, but said the U.S. knows it is "a relatively small number."

Kirby specified that the Iranians "have put trainers and tech support in Crimea, but it's the Russians who are doing the piloting."

“We're going to continue to vigorously enforce all U.S. sanctions on both the Russian and Iranian arms trade. We're going to make it harder for Iran to sell these weapons to Russia. We're going to help the Ukrainians have what they need to defend themselves against these threats."

-ABC News' Justin Gomez

The Justice Department will "continue" to be "relentless" in its pursuit to hold those responsible for war crimes and other atrocities in Ukraine accountable, U.S. Attorney General Merrick Garland said Thursday, while sitting next to his German counterpart, Minister of Justice Christine Lambrecht.

"We are committed to finding ways to expand our cooperation with our German partners in these efforts," Garland said.

Garland also thanked Germany for its help in apprehending a suspect accused of getting sensitive technologies to Russia as part of an indictment announced Wednesday.

The Justice Department charged five individuals including Yury Orekhov, the alleged mastermind behind the plot. Orekhov was arrested in Germany as part of the Justice Department's task force KleptoCapture, which is cracking down on Russian-related crimes as the war in Ukraine continues.

Asked if any U.S. intelligence was compromised after Germany replaced its head of cybersecurity over alleged ties to Russia, Garland didn't answer, saying intelligence sharing is what makes the relationship with Germany so strong.

-ABC News' Luke Barr

The United States is committed to stopping Russia from obtaining foreign weapons, including Iran-made drones, a State Department official said.

Officials from the United States, United Kingdom and France on Thursday raised the issue during a meeting of the U.N. Security Council, State Department spokesman Ned Price said.

"The United States began warning in July that Iran was planning to transfer UAVs to Russia for use in Russia's brutal war against Ukraine, and we now have abundant evidence that these UAVs are being used to strike Ukrainian civilians and critical civilian infrastructure," Price said. "As Iran continues to lie and deny providing weapons to Russia for use in Ukraine, we are committed to working with allies and partners to prevent the transfer of dangerous weaponry to Russia."

He added, "We will not hesitate to use our sanctions and other appropriate tools on all involved in these transfers."

-ABC News' Matthew Seyler

Secretary of State Antony Blinken told "Good Morning America" anchor George Stephanopoulos in a new interview that Russian President Vladimir Putin's declaration of martial law in illegally annexed parts of Ukraine "speaks to his desperation" as Ukrainian forces continue to make progress in rebuffing the invasion.

"Just in the last few weeks, he's tried to mobilize more forces. He's gone through with this sham annexation of Ukrainian territory," Blinken said in a preview from the sit-down, which will air Thursday on "Good Morning America."

-ABC News' Tal Axelrod