Canadian Prime Minister Justin Trudeau made an unannounced visit to Kyiv on Saturday as Ukrainian forces were engaged in fierce fighting against Russia on the southern front line.

Trudeau, who is due to meet Ukrainian President Volodymyr Zelensky later on Saturday, placed flowers by a wall displaying the faces of soldiers killed in combat while a military orchestra played, AFP journalists saw.

He also visited an open-air exhibition featuring destroyed military vehicles.

Ukraine's Deputy Defence Minister Oleksandr Polishchuk handed Trudeau a container holding shrapnel from a rocket that fell on the Black Sea port city of Odesa.

A group of Ukrainian soldiers who received training in Canada spoke with Trudeau. One of them, Colonel Petro Ostapchuk, told reporters the troops received training for elite shooters, engineers and young commanders. 

"It's a great privilege to meet the prime minister," he said.

Canada, which hosts a large Ukrainian diaspora, has been one of Kyiv's key allies since the Russian invasion.

Ottawa has provided Ukraine with significant military aid, trained more than 36,000 soldiers and adopted sanctions against Moscow.

Trudeau's visit comes as Russia has reported thwarting Ukrainian attacks in the east and south that some observers have interpreted as the start of Kyiv's expected large-scale counter-offensive.

Ukraine has so far minimised the importance of the attacks.