Hundreds of mourners packed a Kyiv church on Tuesday for the funeral of volunteers killed on a sabotage mission in Russia, following a spate of attacks along  Moscow's frontier.

Mourners, many in camouflage and covering their faces, attended a service in a central church for four men -- one still a teenager -- killed in December during an incursion into Russia's southern Bryansk region. Russia's FSB security service announced it had killed the men, saying they were armed with rifles and explosives. Russian media reported their bodies were handed over this month.

The men's coffins were draped with the banner of a nationalist battalion called Bratstvo, or Brotherhood, created on the basis of a party of the same name. They were in "one of the reconnaissance sabotage groups of Bratstvo that take part in raids at the enemy's rear, both in the occupied territories... and on Russian soil," the leader of the Brotherhood party Dmytro Korchynsky told AFP outside the church.

"They were killed during one of those raids." Korchynsky said the battalion when in Russia acts "at its own risk" and does not coordinate with Kyiv's armed forces. Crowds then came to pay final respects to the men --- 34-year-old Yuriy Gorovets, 32-year-old Maksym Mykhailov, 34-year-old Taras Karpiuk and 19-year-old Bogdan Lyagov -- on Kyiv's central Independence Square.

Ukraine's armed forces officially do not fight against Moscow beyond the country's borders. There have however been major incidents including an explosion in October on a bridge Russia built to annexed Crimea. Belarusian opposition activists announced late February that partisans had destroyed a Russian military plane near the capital Minsk.

Last week Moscow claimed "Ukrainian nationalists" had crossed into the Bryansk region and killed two civilians, while Kyiv dismissed the allegation as a "deliberate provocation".

Korchynsky said Bratstvo carries out various types of operations including "blowing things up". The four men killed "got past the border guards and went deeper into Russian territory and there they were somehow discovered and engaged in combat with the enemy," he said. "The Russians claim that they (the Ukrainians) were surrounded and were offered a chance to surrender," he added, calling any refusal to surrender "normal for us".

Last week, Russia reported that a group of Ukrainian combatants had crossed into Bryansk region and committed a "terrorist attack". A far-right group of Russian nationalists fighting on Ukraine's side, called the Russian Volunteer Corps, claimed on social media that they had entered the region. The group, whose leader has been linked to football hooliganism and used to run MMA events, told AFP it would not comment on what happened. Russia's FSB this week also accused the same group of attempting to assassinate a Kremlin-linked businessman. The group declined to comment.