The UK government on Wednesday warned that Russia could target civilian shipping in the Black Sea by laying sea mines on the approaches to Ukrainian ports and then blaming Ukraine.

London last month said that Russian forces targeted a civilian cargo ship in the region with "multiple missiles" but they were successfully intercepted by air defences.

Citing declassified intelligence, it said there was a risk of attack to cargo ships travelling through Ukraine's "humanitarian corridor" to deter Ukrainian grain exports.

"Russia's pernicious targeting of civilian shipping in the Black Sea demonstrates Putin's total disregard for civilian lives and the needs of the world's most vulnerable," said Foreign Secretary James Cleverly.

"The world is watching -- and we see right through Russia's cynical attempts to lay blame on Ukraine for their attacks."

Cleverly's department said by releasing its assessment of intelligence reports, "the UK seeks to expose Russia's tactics to deter any such incident from occurring".

Russia withdrew from the Black Sea Grain Initiative in July.

Since then, Russian attacks have damaged 130 port infrastructure facilities and destroyed nearly 300,000 tonnes of grain -- enough to feed more than one million people for a year, the UK said.

On Wednesday, Prime Minister Rishi Sunak reaffirmed UK backing for Ukraine and President Volodymyr Zelensky, urging wavering Western allies to maintain weapons supplies.

"If we give President Zelensky the tools, the Ukrainians will finish the job," he told his ruling Conservative party's annual conference in Manchester, northwest England.