The US is "rushing ammunition, armoured vehicles, missiles [and] air defences" to Ukraine's front line, Anthony Blinken has said.
During a trip to Ukraine, the US secretary of state announced that $2bn would be spent to speed up delivery.
His comments came as Russian advances forced Ukrainian to retreat from several villages in the Kharkiv region.
Ukraine's President Volodymyr Zelensky has cancelled all his foreign trips to concentrate on the new incursion.
Speaking in Kyiv on Wednesday, Mr Blinken said weapons would be rushed to the front lines to "protect soldier, to protect citizens".
He said that the air defence systems, for which Ukraine has repeatedly pleaded for, was "a matter of both the urgency and priority".
Speaking at the same news conference, Ukrainian Foreign Minister, Dmytro Kuleba, said the weapons needed to get to the front lines quickly.
"Everything must happen fast, weapons must come fast so that we can disrupt Russia's offensive plans in Ukraine and prevent its aggressive plans against the rest of Europe and the Euro-Atlantic community," he said.
Last month, the US finally approved a $61bn (£49bn) military aid package, after months of political wrangling. But weeks on, barely any of it has arrived despite Ukraine's pleas for help.
The state department said the $2bn being spent is drawn from security funding recently passed by Congress and previously approved funds.
On Wednesday, Mr Blinken also mentioned the Israel-Gaza war, saying that Israel needs "a clear, concrete plan" for the future of Gaza, to avoid "anarchy and a vacuum that's likely to be filled by chaos".
The US has been "very clear" that it does "not support an Israeli reoccupation", he said, not does the US support Hamas being in control in Gaza.
During his visit, Mr Blinken also toured a grain export facility and a company that makes prosthetic limbs.
