Secretary-General Jens Stoltenberg was visiting Washington where he met Republican lawmakers as an impasse in Congress holds up new US aid to Ukraine, which President Joe Biden argues is critical to combatting Russia's nearly two-year-old invasion.

Speaking at the conservative Heritage Foundation, Stoltenberg made an economic argument, saying that US arms manufacturers have exported $120 billion to European allies and Canada over the past two years.

"What you produce keeps people safe. What allies buy keeps American business strong. So NATO is a good deal for the United States," the alliance chief said.

He said that only a "small fraction" of US annual military spending had destroyed "a substantial part of Russia's combat capacity."

"Supporting Ukraine is not a charity. It is an investment in our own security," he said.

"Ukraine must prevail. And it can -- but it needs our continued help."

Stoltenberg will later travel to the deeply Republican state of Alabama to see a Lockheed Martin plant that has produced Javelin anti-armor weapons for Ukraine.

Trump, the front-runner for the Republican nomination to challenge Biden in November, has voiced misgivings about Ukraine aid and NATO.

The former president has mused about withdrawing from NATO and said it was unfair to commit the United States to defend other members of the 31-nation alliance.

He also voiced admiration for Russian President Vladimir Putin and questioned whether Ukraine can emerge victorious from the invasion.

Stoltenberg met Tuesday with Washington's top Republican, House Speaker Mike Johnson. In a joint statement issued by NATO, the two said that the alliance was "good for Europe and good for the United States."

"We discussed the importance of sending a clear, decisive message to President Putin that he will not win his war of aggression in Ukraine," it said.

But Congress remains at an impasse, with Republicans refusing to approve $61 billion for Ukraine requested by Biden as they press for harsher measures against migrants at the Mexican border.