Ukrainian troops are starting to run out of ammunition as a huge package of US war aid requested by President Joe Biden remains blocked by Republicans in Congress, a top White House official said on Wednesday.

National Security Adviser Jake Sullivan urged the Republican-led US House to follow the lead of the Senate and quickly pass the multi-billion-dollar assistance package for Ukraine's fight against Russia.

"We cannot afford to wait any longer. Every day comes at a cost to the people of Ukraine and to the national security interests of the United States of America," Sullivan told reporters in a briefing at the White House.

"The stakes are getting higher. The costs of inaction are also getting higher every day," he added.

"We've been increasingly getting reports of Ukrainian troops rationing, or even running out of ammunition on the front lines, as Russian forces continue to attack both on the ground, and from trying to wear down Ukrainian air defences."

Sullivan said US allies and adversaries alike were "watching this closely."

The national security adviser's appearance at the podium came a day after Biden made a televised address to urge Republicans in the House of Representatives to pass the bill.

Biden also slammed his likely election rival Donald Trump for "dumb" threats to encourage Russia to invade Nato countries that don't meet their financial commitments as he sought to ram home the message to Republicans.

Ukraine's new army chief said earlier that the situation on the front line with Russia was "extremely difficult" as the delays to US military aid cast a shadow over the war effort. (AFP)