The US State Department condemned what it said was the "deadliest one-day period in 2017" following the deaths of nine Ukrainian soldiers during fighting in the eastern regions of Donetsk and Mariupol on Thursday.

State Department spokeswoman Heather Nauert called for "Russia and the forces that it arms, trains and leads in the east, to immediately observe the ceasefire," referring to accusations of the Kremlin's backing for the rebels who have led a separatist insurgency in several of the country's eastern regions since 2014.

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Nine deaths confirmed

Ukrainian military spokesman Col. Andriy Lysenko said four of the deaths happened during tank and mortar fire near Krasnohorivka, in the Donetsk region. Another was killed in rebel firing on Novogorodsky, also in Donetsk.

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One soldier was killed during rebel fire in the Mariupol region by the Azov Sea, while three others were killed by a mine in the same area, Lysenko added.

Violence has flared for several days after a top rebel leader announced a plan to form a new "state" that Kiev warned could put a long-stalled peace plan further in jeopardy.

New state idea falls flat

The proposal - which would see a new country declared called Malorossiya (Little Russia) - has since been rejected by other separatist leaders. But the call sparked fears that a two-year-old ceasefire would be further undermined.

A truce was agreed in 2015 following more than a year of bloody battles and the deaths of around 10,000 people but it was never fully implemented. Although full-scale offensives have stopped, both sides report artillery fire or small clashes almost daily.

Moscow denies accusations from Ukraine and the West that it has armed the separatists - despite overwhelming evidence to the contrary. The rebels are aligned to Moscow through historic connections when Ukraine was part of the Soviet Union.

The conflict evolved out of months of street protests across Ukraine that saw the Moscow-backed president Viktor Yanukovych forced from office in February 2014. Shortly afterwards, Russia annexed the Ukrainian region of Crimea before the conflict spread east.

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mm/bk (AP, AFP)