U.S. officials are warning China against supplying Russia with arms and ammunition, as Moscow allegedly struggles to gain ground in Ukraine despite deploying almost the entirety of its ground forces in its smaller neighbor. Concerns that China was considering providing lethal assistance to Russia first surfaced in meetings between officials late last year and early this year, officials said. U.S. officials put their Chinese counterparts on notice in videoconferences and at in-person meetings that China is “nearing a red line” in assisting Russia’s war, the officials said.