Gunmen shot dead 11 people at a Russian military training ground on Saturday, the Defence Ministry said, in the latest blow to President Vladimir Putin's forces since the invasion of Ukraine.

RIA news agency cited the ministry as saying 15 other people were wounded in the shooting on Saturday in the southwestern Belgorod region that borders Ukraine when two men gunned down a group who had volunteered to take part in the war.

It said the two assailants - nationals from an unspecified former Soviet republic - had been shot dead. Some Russian independent media outlets reported that the number of casualties was higher than the official figures.

The attack took place a week after a blast damaged a bridge in Crimea, the peninsula annexed by Russia from Ukraine in 2014. Earlier in the war, Russia's flagship in the Black Sea blew up and sank.

"During a firearms training session with individuals who voluntarily expressed a desire to participate in the special military operation (against Ukraine), the terrorists opened fire with small arms on the personnel of the unit," RIA cited a defence ministry statement as saying.

Just a day earlier, Putin said Russia should be finished calling up reservists in two weeks, promising an end to a divisive mobilisation that has seen hundreds of thousands of men summoned to fight in Ukraine and huge numbers flee the country.

Oleksiy Arestovych, an advisor to Ukrainian President Volodymyr Zelenskiy, said in a YouTube interview that the attackers were from the Central Asian nation of Tajikistan and had opened fire on the others after an argument over religion.

Tajikistan is a predominantly Muslim nation, while around half of Russians follow various branches of Christianity. The Russian ministry had said the attackers were from a nation in the Commonwealth of Independent States, which groups nine ex-Soviet republics, including Tajikistan.

Reuters was not immediately able to confirm the comments by Arestovych, a prominent commentator on the war, or independently verify casualty numbers and other details of the incident.

The independent Russian news website Sota Vision said the attack occurred in the small town of Soloti, close to the Ukrainian border and about 105km southeast of Belgorod.

Zelenskiy said on Saturday that Ukrainian troops were still holding the strategic eastern town of Bakhmut despite repeated Russian attacks while the situation in the larger Donbas region remained very difficult.

Although Ukrainian troops have recaptured thousands of square kilometres of land in recent offensives in the east and south, officials say progress is likely to slow once Kyiv's forces meet more determined resistance.

Ukrainian forces and civilians are relying on Starlink internet service provided by Elon Musk's SpaceX rocket company. Musk said on Friday he could no longer afford to fund the service but on Saturday said he would continue to do so.

Zelenskiy said almost 65,000 Russians had been killed so far since the Feb 24 invasion, a figure far higher than Moscow's official Sept 21 estimate of 5,937 dead. In August the Pentagon said Russia has suffered between 70,000 and 80,000 casualties, either killed or wounded.

Russian missile, drone attacks

Putin ordered the mobilisation three weeks ago, part of a response to Russian battlefield defeats in Ukraine. He has also proclaimed the annexation of four partially occupied Ukrainian provinces and threatened to use nuclear weapons.

Zelenskiy, speaking in an evening address, also said Russian missiles and drones had continued to hit Ukrainian cities, causing destruction and casualties.

Kyiv said on Friday that it expected the United States and Germany to deliver sophisticated anti-aircraft systems this month to help defend against the missiles.

Fighting is particularly intense in the eastern Donetsk and Luhansk provinces bordering Russia. Together they make up the larger industrial Donbas, which Moscow has yet to fully capture.

Russian forces have repeatedly tried to seize Bakhmut, which sits on a main road leading to the cities of Sloviansk and Kramatorsk. Both are situated in the Donetsk region.

Separately, the Ukrainian armed forces' general staff said in a Facebook post that troops had on Saturday repelled a total of 11 separate Russian attacks near Kramatorsk, Bakhmut and the town of Avdiivka, just to the north of Donetsk.

- Reuters