Russia said on Thursday it was clear the United States was not willing to address its main security concerns in their standoff over Ukraine, but both sides kept the door open to further dialogue.

The United States and NATO submitted written responses on Wednesday to Russia's demands for a redrawing of post-Cold War security arrangements in Europe since it massed troops near Ukraine, prompting Western fears of an invasion and new US pledges of defence support.

Kremlin spokesperson Dmitry Peskov said Moscow needed time for review and would not rush to conclusions, but that US and NATO statements describing Russia's main demands as unacceptable did not leave much room for optimism.

"Based on what our (US and NATO) colleagues said yesterday...