British Defence Secretary Ben Wallace said on Thursday it would be legitimate for Ukrainian forces to target Russian logistics but they were unlikely to use British weapons.
Moscow has accused London of provoking Ukraine to strike targets in Russia, saying there would be an immediate "proportional response" if it continued.
"If Ukraine did choose to target logistics infrastructure for the Russian army, that would be legitimate under international law," Wallace told BBC TV.
He said any long-range weapons were unlikely to come from Britain however, because Ukrainian forces tend to use mobile launchers while the British army would deliver them from the air or sea.
"It is certainly the case that Britain is assisting and finding artillery for Ukraine, which it is mainly using within Ukraine on Russian forces," he said.
Wallace also denied that NATO was locked in a proxy war with Russia but said the West would provide increasing support to Ukraine if the Russian attacks continued. "Sometimes that will include planes and tanks," he told Times Radio.
He said he expected Russian President Vladimir Putin to dig in in Ukraine, like a "cancerous growth" but said the Russian president also appeared to want to go further.
"You can see in his current statements he is, in almost desperation, trying to broaden this either with threats or indeed, with potential false flags or attacks," he said.
(Reporting by Muvija M and Kate Holton; editing by Michael Holden)