More than eight months into the invasion, officials are pessimistic that Russia will sit down for meaningful peace talks any time soon.
Allies of Ukraine are providing cold-weather gear to Ukrainian forces as winter approaches.
A prolonged stalemate would frustrate the hopes of Ukrainian President Volodymyr Zelenskyy that his country would gain the upper hand by December.
Although Ukraine made notable gains in an autumn counter-offensive, its progress on the battlefield in recent weeks has been slow.
One western official said on Wednesday that there was no end to the war in sight, with both sides “digging in now for the long winter ahead, as temperatures start to fall”.
“This is going to continue to be a long, difficult, bloody conflict,” the official said.
The official added that existing Russian offers to negotiate were not serious and amounted to demanding Ukraine's surrender.
On the Kremlin's thinking, the official said: “I do not think we are seeing signs that there is any sort of serious reappraisal. We're not seeing signs yet that there might be a Plan B.”
The US said last week that it did not see either Ukraine or Russia as likely to return to the negotiating table soon.
Mr Zelenskyy has signalled that his focus is the counter-offensive and that he does not want to negotiate at present, Pentagon official John Kirby said.
“He gets to determine what, or what he is not, willing to negotiate with the Russians. But we’re just not there yet,” Mr Kirby said.
Russia and Ukraine held tentative peace talks in the early weeks of the war, in which Mr Zelenskyy indicated he could accept a form of neutrality in lieu of Nato membership.
But the talks came to a standstill after civilian massacres were discovered in towns near Kyiv, such as Bucha, and have yet to be revived.
Turkey, which helped to broker a grain deal signed separately by Russia and Ukraine, has offered itself as a mediator in potential future talks.
The western official said on Wednesday that Russia was hoping to “terrify and cow the Ukrainian people” by attacking key infrastructure in air strikes.
Ukraine and its western allies say Russian armed forces are using Iranian-made drones as part of their attacks — something Moscow denies.
Russia has hit back with claims that Britain is linked to the Nord Stream pipeline explosions or attacks on the Crimean peninsula, which London in turn denies.
Ukraine conflict — in pictures
Moscow said on Wednesday it would call in the British ambassador “in the near future” to hand over supposed evidence linking the UK to an attack on its Black Sea fleet.
Britain on Wednesday announced further sanctions against steel and petrochemical tycoons it said were involved in funding Russia's invasion of Ukraine.
The four oligarchs, Albert Shigabutdinov, Airat Shaimiev, Alexander Abramov and Alexander Frolov, are being hit with asset freezes and travel bans, as has happened with more than 100 others.
“By targeting these individuals, we are ramping up the economic pressure on [Russian President Vladimir] Putin and will continue to do so until Ukraine prevails,” UK Foreign Secretary James Cleverly said.
The western official said outrage over the invasion was still being communicated directly to the Russian leadership even as it shows little sign of changing course.
“The best thing for Ukraine is for Russia to stop this, so I think we need to keep delivering that message,” the official said. “We need to keep reminding them that they do have a choice.”