Throughout the war, Russia has repeatedly complained about Western countries arming Ukraine.
Putin and other top Kremlin officials have warned this could drag the West into direct conflict.
Ukrainian advances, meanwhile, have seen it claim a massive haul of abandoned Russian weaponry.
Throughout Russia's war in Ukraine, which has lasted over seven months with no immediate end in sight, Kyiv has received tons of military assistance from a number of Western countries - including billions of dollars in weapons and other aid from the US. But Ukraine is also getting a lot of weapons from the enemy.
Russian President Vladimir Putin and other top Russian government officials have long complained about the West and the decision to arm Ukraine. They have made little attempt to hide this frustration, even threatening escalation and warning that the conflict could expand.
But as Ukrainian forces continue their weeks-long counteroffensive, advancing along the war's northeastern front and in the south, retreating Russian troops have left behind mountains of weaponry, equipment, and ammunition in their wake.
According to a new Wall Street Journal report citing open-source intelligence, Ukraine's recent capture of all this Russian weaponry - in addition to what it obtained when Putin's troops retreated from areas near the Ukrainian capital of Kyiv in the spring - has turned Russia into Ukraine's biggest supplier of heavy weapons.
This observation is based purely on quantities, as opposed to the quality of the weapons, the report said. Among the weapons left behind are tanks and other armor, artillery pieces, and various firearms.
During the early days of Ukraine's lightning-fast offensive in the northeast Kharkiv region, Russian troops left behind so much weaponry and ammunition that Ukrainian forces struggled to handle it all. Some Russians abandoned their positions in a hurry, leaving behind their rifles and stealing bicycles to get away.
On top of what Ukrainian troops have obtained from fleeing Russian units, they continue to be supplied by Western countries, much to the dismay of Kremlin leadership.
As early as January, before Putin's unprovoked invasion of Ukraine, as his troops gathered along the border, the Russian leader demanded the US and its allies and partners stop arming Ukraine. During the spring, Russia warned of "unpredictable consequences" if the US continued to deliver weapons to Ukraine. Over the summer, Putin threatened to attack new targets in Ukraine if the West armed the eastern European country with longer-range weapons.
Last month, Russian foreign ministry spokeswoman Maria Zakharova said the US would cross a "red line" if it sent long-range missiles to Ukraine. And as recently as this week, after the Biden administration announced a new $625 million military aid package for Kyiv, two Russian diplomats said the move could bring Russia closer to a direct war with the West.
Top Western officials and heads of state have shown no apparent signs that they will back off weapons deliveries to Ukraine.
"The United States will continue to provide Ukraine with the weapons and equipment to meet its urgent needs on the battlefield, while also building Ukraine's enduring strength to defend its sovereignty over the long term," Deputy Assistant Secretary of Defense for Russia, Ukraine and Eurasia Laura Cooper told reporters at a briefing Tuesday.