China is backing Russia’s war effort in Ukraine by helping Moscow in its biggest military buildup since the Soviet era, providing drone and missile technology, satellite imagery and machine tools, senior U.S. officials said on Friday.

The officials, speaking on condition of anonymity, said U.S. President Joe Biden raised the issue with Chinese President Xi Jinping in their recent phone call and that it is a topic of discussion with U.S. allies in Europe and around the world.

One official said Chinese materials are filing critical gaps in Russia’s defense production cycle and helping Moscow undertake its “most ambitious defense expansion since the Soviet era and on a faster timeline than we believed possible early on in this conflict.”

President Joe Biden has been pressuring Republicans who control the U.S. House of Representatives to approve a major infusion of funding for providing weapons to Ukraine as it struggles to fend off the Russians.

[Edit typo.]