A Russian missile strike on a training exercise in Ukraine's Sumy border region has killed six servicemen and wounded more than 10 others, says Ukraine's National Guard.

Russia's defence ministry had earlier released a video purporting to show an Iskander missile attack on a training camp, and state news agency Tass said up to 70 people had died.

The Sumy region has come under repeated bombardment, and Ukraine launched a months-long occupation of part of Russia's neighbouring Kursk region from there.

The Ukrainian military said the aim of the offensive had been to help create a buffer zone to protect Sumy, but some have complained of the scale of military losses.

Separately, the Kremlin said President Vladimir Putin had visited officials in Kursk the previous day, in his first visit to the region since Ukrainian forces were pushed out.

He met volunteers as well as acting local governor Alexander Khinshtein, and he toured a nearby nuclear power plant, state media reported.

The deadly Russian strike on Ukrainian servicemen in Sumy was confirmed early on Wednesday by Ukraine's National Guard, which said the attack took place while the unit was carrying out exercises at a firing range. The commander was suspended and an official investigation launched, it added.

The unverified Russian military video showed dozens of servicemen walking on a path near a wooded area followed by an explosion and a large plume of smoke.

Research by BBC Verify showed the training camp was targeted in the far north of the Sumy region, some way south of the Russian border.

The attack comes as a heavy blow to the Ukrainian military, and the National Guard said it had previously developed an "algorithm of actions" as well as orders to deal with the threat of air strikes and people gathered in one place.

Ukraine's general staff said this week that it had thwarted Russia's bid to establish a "security zone" in Sumy region, and argued that its Kursk operation had "strategic significance", forcing Russia to divert its "most capable units" to tackle the offensive.

The Kremlin also sent thousands of North Koreans soldiers to the region in a bid to recapture the area.

However, last week a battalion commander called Oleksandr Shyrshyn was quoted as criticising the "moronic tasks" set by the military leadership as well as unjustified losses.

Russia launched its full-scale invasion of Ukraine in February 2022 and has since annexed four eastern regions of Ukraine's sovereign territory, including large areas that remain under Ukrainian control.

Ukraine's Kursk offensive has not succeeded in halting Russia's push to capture more territory in the east, although its advance along the front lines has been very slow.

Ukrainian forces on the eastern flanks say Russia launched 14 attacks overnight towards Druzhba, Petrivka, and Toretsk.

A Ukrainian soldier close to the eastern city of Pokrovsk told the BBC that there has been a major push by Russian forces there too.

He said invading troops had targeted a major road that is used as a supply line, adding that they had been struggling to hold their positions for some time.

Hopes of an imminent ceasefire in the war are fading, despite low-level talks between Ukraine and Russia in Istanbul last Friday.

Although US President Donald Trump suggested the Vatican might mediate further talks, the Vatican said the idea of hosting, or even mediating talks was more a hope for now than any concrete plan.

US Secretary of State Marco Rubio said he expected Russia to present its "broad terms that would allow us to move towards a ceasefire", however Ukraine's Volodymyr Zelensky has said Russia is merely "trying to buy time in order to continue its war and occupation".