President Donald Trump has sent a letter of support to Ukraine praising its courage and saying the United States believes in its future as an independent state, as the country marks the anniversary of declaring itself free of Soviet rule.

"Now is the moment to bring an end to the senseless killing," Trump said in a message published by Zelensky on Sunday.

Potential security guarantees could end the three-year war between Ukraine and Russia, Ukrainian President Volodymyr Zelensky announced on Saturday afternoon.