THE Canadian government is urging its citizens to avoid non-essential travel to Ukraine, in a new advisory citing "Russian aggression."

Moscow has stationed more than 100,000 troops near the border with Ukraine, and the United States said on Friday it feared Russia was preparing a pretext to invade if diplomacy failed to meet its objectives.

"We have changed the risk level for Ukraine to avoid non-essential travel due to ongoing Russian aggression and military buildup in and around the country," the Canadian government said in a travel advisory issued late Saturday.

Canada, with a sizeable and politically influential population of Ukrainian descent, has taken a hard line with Russia since its annexation of Crimea from Ukraine in 2014.

Canadian Foreign Minister Melanie Joly is to visit Kyiv next week to reaffirm Canada's support for Ukrainian sovereignty and reinforce efforts to deter "aggressive actions" by Russia, Ottawa said earlier.

Joly will meet Ukrainian Prime Minister Denys Shmygal and travel to the west of the country to speak to a 200-strong Canadian training mission that has been there since 2015.

Canadian Deputy Foreign Minister Marta Morgan and U.S. Deputy Secretary of State Wendy Sherman spoke on Friday and pledged continued close coordination to deter further Russian aggression against Ukraine and called for Russian de-escalation, a U.S. State Department spokesperson said on Saturday.

In an interview with Canadian broadcaster CBC published Sunday, NATO Secretary General Jens Stoltenberg described Russia as "the aggressor." He told CBC News it was up to Russia to de-escalate and that NATO was willing to sit down again and listen to Moscow's concerns.

Russia denies plans to attack Ukraine, but says it could take unspecified military action unless its demands - including a promise by the NATO alliance never to admit Kyiv - are met.

After talks between the United States, its European allies and Russia ended last week, U.S. officials warned that the risk of a Russian invasion of Ukraine remained high.

Canadian Prime Minister Justin Trudeau spoke to Ukrainian President Volodymyr Zelenskiy on Tuesday and "emphasized that any military incursion into Ukraine would have serious consequences, including coordinated sanctions."

Canada has imposed punitive measures on more than 440 individuals and entities over the annexation of Crimea.