NATO chief Jens Stoltenberg said Thursday a delay in passing new US aid for Ukraine was already hurting Kyiv's forces on the battlefield against Russia.

"We see the impact already of the fact that the US has not been able to make a decision, but I expect the US to be able to make a decision, that the Congress and the House of Representatives will agree continued support to Ukraine," Stoltenberg told journalists ahead of a meeting of NATO defense ministers. 

Ukrainian commanders and Western officials have said that Kyiv's troops are being outgunned by Russia along the front line and are having to use strained supplies of artillery and air defence munition sparingly.

"If we allow President Putin to win, it will not only be bad for the Ukrainians, a tragedy for Ukrainians, but also be dangerous for us. It will make the world even more dangerous and us more vulnerable," Stoltenberg said. 

The head of the Western alliance said he had seen "broad agreement" during a recent visit to Washington on the need to maintain US backing for Kyiv. 

"I count on the US Congress to be able to reflect that support in a decision to sustain the support for Ukraine," he said. 

The US Senate on Tuesday approved a $95 billion package for Ukraine, Israel and Taiwan by a comfortable margin, but House Speaker Mike Johnson, a Donald Trump ally, has refused to put it to a vote in the lower chamber.

Germany's defence minister Boris Pistorius said he would lobby US lawmakers on the issue at the upcoming Munich Security Conference. 

"We have to emphasize our willingness to support Ukraine. This is an issue for all of us," he said. 

The doubts over future US aid for Ukraine come at a moment of heightened worry over American leadership after Republican White House frontrunner Trump rocked NATO by suggesting he wouldn't protect allies that don't spend enough on defence. 

NATO on Wednesday released fresh figures showing that 18 of its 31 member countries are set to meet a target this year of spending 2 percent of gross domestic product. 

French defence minister Sebastien Lecornu said Thursday that France would join European powers such as Germany in reaching that goal this year. 

Stoltenberg said it was a "good thing" that NATO's European members were ramping up defense expenditure in the face of Russia's aggression, but warned against trying to go it alone without the US. 

"That's not an alternative to NATO. That is actually a way to strengthen NATO," he said. 

Stoltenberg insisted that the alliance's Article 5 commitment to defend any ally if attacked remained the "heart" of NATO. 

"And of course that applies for all allies," he said.