U.S. watches as Ukrainian forces push into Russia with U.S.-built arms

(CNN) — The United States has watched closely as Ukrainian forces pushed deeper into Russian territory, powered in part by U.S.-provided arms and equipment that have raised questions about whether Kyiv has abided by the Biden administration’s restrictions on the use of American weaponry.

Ukraine has used U.S. weaponry and vehicles as part of its offensive, according to officials, even though the Biden administration did put boundaries on the use of U.S. weaponry in Russia, according to two U.S. officials. The officials would not detail the exact boundaries the United States drew but said Ukraine had not “technically” breached the U.S.-imposed limitations.

U.S.-made Humvees were seen entering Russia’s Kursk oblast through the Ukraine-Russia border crossing near Sudzha, Russia, on Tuesday in a Reuters photograph.

Ukraine has also used HIMARS rocket launchers as part of the operation, officials said, but the vehicles appear to have remained in Ukrainian territory, using the range of its GMLRS missile to hit Russia.

The Russian Ministry of Defense released drone video on Thursday, showing a HIMARS rocket-launching vehicle firing missiles in a wooded area just under five miles from the Russia-Ukraine border. The U.S. restrictions on the use of long-range ATACMS missile has not changed, U.S. officials said. Ukraine has used the missiles to strike Russian air defense systems in Russian-occupied Crimea.

It’s unclear to what extent the U.S. vehicles have been used so far in the operation in Kursk.

Pro-Russian Telegram channels have circulated images claiming to show a destroyed Ukrainian convoy in the Kursk oblast. CNN is unable to determine the exact location where the photos were taken, but one of the destroyed vehicles is a US-made Cougar MRAP.

One U.S. official said Ukraine’s military has used U.S.-made Stryker combat vehicles capable of moving infantry across a battlefield. Video released by the Russian Ministry of Defense claims to show a military strike hitting a U.S.-made Stryker vehicle in a wooded area, but CNN could not verify the claim.

On Friday, Ukrainian army chief Oleksandr Syrskyi said the Ukrainian military, already some 35 kilometers into Russian territory, is still advancing “in some areas from 1 to 3 kilometers.” The operation has put the Kremlin on the defensive, forcing Russia to bring in troops from the front lines to repel the ongoing Ukrainian raid.

“Overall, the situation is under control. All activities are being carried out according to plan,” Syrskyi said.

The United States drew a box north of Ukraine from which Russian forces were carrying out attacks and instructed Kyiv to keep their use of American weapons contained to that area, according to two U.S. officials. The officials would not describe the exact location of the geographic box or its size, but they said Ukraine’s incursion has remained within its borders, and the operation has not violated U.S. policy regarding the use of American weapons on Russia.

Because the operation has remained within the agreed-upon area, officials say it has not “technically” breached U.S.-imposed limitations. One official said the Ukrainian offensive follows the “letter of the law” when it comes to U.S. policy but acknowledged it’s “not what we had in mind.”

The United States has not publicly taken a position on Ukraine’s maneuver, which officials say has had a “surprising” amount of success.

The Biden administration has not instructed Kyiv to pull back its forces into Ukraine, nor has it thrown its full support behind the operation. Instead, the administration has been careful in stating openly that it will continue to back Ukraine’s ability to defend itself.

“They’re going to continue to have American support for the defense of their territory for defeating Russian aggression,” John Kirby, strategic communications coordinator for the National Security Council, told CNN Friday.

Equally surprising, officials said, is the Kremlin’s slow response to the incursion, the first time Russia has lost sovereign territory since invading Ukraine more than two years ago.

Russia appears to have diverted at least several thousand troops away from front-line fighting in occupied Ukraine in order to address the territorial loss in the Kursk region. One U.S. official said Russia will likely end up having to send more troops to the area to strengthen its own defenses, as the United States watches to see where Russia will find the manpower.

Meanwhile, senior U.S. officials have tried to gain a better understanding of Kyiv’s goal in carrying out the offensive into Russia. Ukraine did not inform the United States in advance of the incursion, and the United States is still not clear on what the long-term goal is for the operation.

On Friday, an aide to Ukrainian President Volodymyr Zelensky said the cross-border incursion is aimed at creating a “fair negotiation process” and inflicting “significant tactical defeats” on Russia.

“Negative changes in the psychological state of the Russian population will be another argument for the start of negotiations,” said Mykhailo Podolyak, adviser to the Head of the Office of the President of Ukraine, on social media.

Russia’s reaction may be delayed, officials said, but there is little doubt that a response is coming. Ukraine says it has seized approximately 400 square miles of territory inside Russia, forcing Russian authorities to declare emergencies and carry out large-scale evacuations.

U.S. Defense Secretary Lloyd Austin spoke with his Ukrainian counterpart, Rustem Umerov, Wednesday and tried to learn more about the objectives of the offensive, launched nearly two weeks ago.