Western officials say Moscow has been surprised by the ferocious resistance being put up by Ukrainian forces and that it has thwarted the Kremlin's plan for a lightning advance.

"The Ukrainian military is fighting with extraordinary courage and skill against Russian forced that significantly outnumber them and outgun them," Milley, the chairman of the US Joint Chiefs of Staff, told journalists on a visit to NATO in Brussels.

"They are performing extraordinarily well on a very difficult sort of battlefield."

The Russian military has admitted that almost 500 of its soldiers have been killed in the week since President Vladimir Putin ordered the invasion.

Ukraine claims to have killed around 9,000 Russian troops.

Kyiv's forces "are fighting on their own terrain, they know the turf -- a significant advantage of course is that they have the Ukrainian people," Milley said.

"The Ukrainian people are the eyes and the ears of the Ukrainian army right now. So they get the best intelligence system in the world, because they have the entire population on their side."

Milley was in Brussels at the start of a visit to Europe designed to show Washington's "determination" to defend NATO's eastern flank in the face of Russia's aggression.

Allies, spearheaded by the US, have rushed thousands of troops to countries closer to Russia as fighting rages in non-NATO member Ukraine.

Milley was set to meet US troops in Germany as the alliance looks to make clear to Moscow that it will trigger NATO's collective defence clause if any member is attacked.

"Their mission is critical to the contribution that we are making to NATO, but it's directly related to the US national interests, for NATO to deter any further territorial aggression by Russia against Article Five countries," Milley said.

NATO allies are supplying arms to Ukraine to fight the Russian forces, but insist they will not intervene directly in the conflict.

They say any use of NATO forces would trigger a direct conflict with Russia that could spiral into nuclear war.

That comes in the face of persistent calls from some quarters for the alliance to try to impose a no-fly zone over Ukraine to stop Moscow's bombing.

"We're not part of the conflict, we will not be part of the conflict," NATO chief Jens Stoltenberg said Wednesday at a meeting with Iceland's prime minister.

"NATO will not be on the ground with forces or soldiers in Ukraine and will not be in the airspace of Ukraine."