The Russia-Ukraine conflict could last a "number of years" and the world needs to be prepared for Moscow "to seek to use even worse weapons", British Foreign Secretary Liz Truss warned Sunday.
"I fear this will be a long haul, this could be a number of years," Truss told Sky News.
"Russia have strong forces and we know the Ukrainians are brave, they are determined to stand up for their sovereignty and territorial integrity and they are determined to fight," she said.
The minister said that intelligence showed that Ukrainian forces were "continuing to resist Russian advances" and that there had not been "significant changes" overnight.
But she warned Russian President Vladimir Putin could deploy more deadly weapons.
"This could well be the beginning of the end for Putin and I fear that he is determined to use the most unsavoury means in this war."
"I fear this conflict could be very, very bloody. We do need to be prepared for Russia to seek to use even worse weapons," she added.
However, Putin "should be aware the International Criminal Court is already looking at what is happening in Ukraine and there will be serious consequences for him personally," she told Sky News.
Western allies on Saturday agreed on a new volley of financial sanctions against Russia over its invasion of Ukraine, including the exclusion of a number of Russian banks from the SWIFT interbank system.
The allies also agreed to impose restrictive measures to prevent the Russian central bank from "using international financial transactions to prop up the ruble", a senior US official said.
Britain has provided Ukraine with lethal defence weapons and applied sanctions to Kremlin-linked tycoons and businesses.
The foreign minister said that Britain would continue to supply defence weapons, but cautioned that the sanctions "will take time" to become effective and that they could only work through their "hit list" of oligarchs as fast as the legal process would allow them to.
"We've already had letters to the Foreign Office, from lawyers, threatening us, so we have to make sure the cases are properly prepared and that we have the right evidence before we sanction these individuals," she told the Sunday Times.
"That is why we're taking it step by step, but we are working through that hit list and we will continue to sanction new oligarchs every few weeks."
British Prime Minister Boris Johnson spoke to Ukraine President Volodymyr Zelensky late Saturday, with the pair agreeing that the world needs to isolate Russia "completely diplomatically and financially," Downing Street said.
The pair welcomed international moves towards excluding Russia from the SWIFT system.
Johnson praised the "incredible heroism and bravery of President Zelensky and the Ukrainian people", and both leaders said that Putin's forces were "being met with a greater Ukrainian resistance than he calculated on".
Johnson and Zelensky also shared their "mutual concern" about the role Belarus has played so far in Russia's invasion.
(Except for the headline, this story has not been edited by NDTV staff and is published from a syndicated feed.)