After billions in US aid have been sent to Ukraine since the invasion nearly two years ago, many Republican lawmakers have grown reluctant to keep supporting Kiev, saying it lacks a clear end game as the fighting against President Vladimir Putin's forces grinds on.

President Joe Biden has asked Congress to approve $61 billion in new aid to Ukraine. But the talks have bogged down as Republican lawmakers -- furious over record illegal flows over the US border with Mexico -- demand major changes in US immigration and border control policy in exchange for approving more money for Ukraine.

NATO secretary-general Stoltenberg plans to make the case in Washington this week for continued aid to Ukraine.

"What matters is that Ukraine gets continued support, because we need to realize that this is closely watched in Beijing," Stoltenberg said on Fox News.

Analysts say China, which claims the self-ruled island of Taiwan and has not ruled out the use of force to achieve this control, is watching to see if once-strong Western support for Ukraine is now petering out.

If Ukraine were abandoned by the US and its allies, China might be tempted to take military action to seize control of Taiwan, these analysts warn.

"So it's not only making Europe more vulnerable, but all of us, also the United States, more vulnerable, if Putin gets what he wants in Ukraine," Stoltenberg added.

He said the agreement being negotiated in the US Congress is "a good deal." US aid to Ukraine, Stoltenberg said, has been just a fraction of the Pentagon budget, and yet "we have been able to destroy and degrade the Russian army substantially."

"And therefore we should continue to do so," he said.

US aid to Ukraine also helps American workers, because the money is used to buy weapons made in the United States, the NATO chief said.

Stoltenberg is due to meet Monday with Secretary of State Antony Blinken, Defense Secretary Lloyd Austin and national security advisor Jake Sullivan.

On Tuesday he is scheduled to meet Republican and Democratic lawmakers involved in the Ukraine aid debate.

Donald Trump, the almost certain Republican candidate in the November presidential election, and who has often spoken fondly of Putin, is urging Republican lawmakers to reject the immigration accord being negotiated in Congress -- which would also torpedo aid for Ukraine.