WRAPUP 6-Ukraine says troops holding onto Sievierodonetsk, advance in south

(Adds death sentence for two Britons and a Moroccan captured while fighting for Ukraine)

* Russia focusing efforts on last Luhansk cities held by Ukraine

* Ukraine claims advances in southern Kherson region

* Separatist Donbas region sentences Britons, Moroccan to death

* Rising concern over food crisis as Russia blockades ports

By Pavel Polityuk and Abdelaziz Boumzar

KYIV/SLOVIANSK, Ukraine, June 9 (Reuters) - Ukrainian forces claimed on Thursday to have pushed forward in intense street fighting in the eastern city of Sievierodonetsk, but said their only hope to turn the tide was more artillery to offset Russia's massive firepower.

In the south, Ukraine's defence ministry said it had captured new ground in a counter-attack in Kherson province, targeting the biggest swathe of territory Russia has seized since its invasion in February.

The battle amid the ruins of Sievierodonetsk, a small industrial city, has become one of the war's bloodiest, with Russia concentrating its invasion force there. Both sides say they have inflicted massive casualties.

Sievierodonetsk and its twin city Lysychansk on the opposite bank of the Siverskyi Donets river are the last Ukrainian-controlled parts of Luhansk province, which Moscow is determined to seize as one of its principal war objectives.

Russian forces are focusing all of their might in the area, Ukraine's Security Council Secretary Oleksiy Danilov told Reuters in an interview on Thursday.

"They don't spare their people, they're just sending men like cannon fodder," he said. "They are shelling our military day and night."

Two Britons and a Moroccan who were captured while fighting for Ukraine were sentenced to death on Thursday by a court in the self-proclaimed Donetsk People's Republic (DPR), one of Russia's proxies in eastern Ukraine, Russian news agencies reported.

STREET FIGHTING

In a rare update from Sievierodonetsk, the commander of Ukraine's Svoboda National Guard Battalion, Petro Kusyk, said Ukrainians were drawing the Russians into street fighting to neutralise Russia's artillery advantage.

"Yesterday was successful for us - we launched a counteroffensive and in some areas we managed to push them back one or two blocks. In others they pushed us back, but just by a building or two," he said in a televised interview.

"Yesterday the occupiers suffered serious losses - if every day were like yesterday, this would all be over soon."

But he said his forces were suffering from a "catastrophic" lack of counter-battery artillery to fire back at Russia's guns, and getting such weapons would transform the battlefield.