A top ally of US President Donald Trump has admitted telling a Ukrainian official that American military aid was contingent on Kiev announcing an investigation into Trump's Democratic rival Joe Biden.

While Ukraine is not a NATO member, the alliance provides political and practical support to Kiev and has stepped up ties since Russia annexed Crimea in 2014.

Stoltenberg refused to comment directly on the evidence given by US ambassador to the EU Gordon Sondland to the House impeachment inquiry into Trump but said NATO stood firm in its commitment to Ukraine.

"I will not go into the process taking place in the US Congress. What I will do is make sure that all NATO allies continue to provide support to Ukraine and that is exactly what they are doing," Stoltenberg told reporters.

"We are actually stepping up after aggressive actions against Ukrainian ships in the Black Sea. NATO allies including the United States prove every day that we stand in solidarity with Ukraine."

Last week Stoltenberg visited Ukraine along with NATO ambassadors, making a speech to parliament and visiting four alliance naval vessels docked in Odessa.

Sondland said he told a senior Ukraine official that military aid would likely not be released until Kiev made clear it would investigate Biden and his son's ties to Ukraine energy firm Burisma.

The White House has repeatedly denied there was any link between the aid and any demands by Trump.