Their assessment was that an “utterly horrific” attack on the capital of Kyiv could come as Russian forces overcome the logistical issues suspected of delaying their attacks.
The warnings came on Wednesday as Ukrainian President Volodymyr Zelensky said children were among those under the wreckage of a “direct strike” at a maternity hospital in the southern port city of Mariupol.
Foreign minister Dmytro Kuleba called it a “petrifying war crime”, as he pleaded for allies to supply Ukraine with aircraft.
Boris Johnson said there are “few things more depraved than targeting the vulnerable and defenceless” as he reiterated that Britain was exploring more support to help Ukraine defend against airstrikes.
One Western official said: “I think we’ve got good reason to be concerned about possible use of non-conventional weapons, partly because of what we’ve seen has happened in other theatres.
“As I’ve mentioned before, for example, what we’ve seen in Syria, partly because we’ve seen a bit of setting the scene for that in the false flag claims that are coming out, and other indications as well.
“So it’s a serious concern for us.”
Defence Secretary Ben Wallace said that the UK has already handed Kyiv 3,615 anti-tank weapons, known as NLAws, or new light-antitank weapons.
He said a “small consignment” of Javelin anti-tank missiles will follow, while ministers consider whether to send Starstreak high-velocity anti-air missiles.
But a Polish proposal to hand its MiG-29 fighter jets to a US military base in Germany, with the expectation they would be handed to Ukrainian pilots, was dismissed in Washington.
Downing Street said the Prime Minister, who met with his Polish counterpart in London Mateusz Morawiecki on Tuesday, believed it “wouldn’t be tenable for Nato pilots and Nato-badged jets to be shooting down Russian aircraft”.
Allies fear that such a move would risk provoking a wider conflict, and the Western official said the difference between the weapons is that the existing support is “defensive”.
“You can’t use a Nlaw to attack Moscow, you can’t use the Stingers to launch over a long range. These are weapons that can only really be used defensively,” they said.
“Some Nato members would have seen that (Polish aircraft) as very different and possibly likely to provoke a different kind of Russian reaction. So that’s a matter for the US and the Poles but I don’t think that’s going anywhere.”
The official warned that Russian troops on the ground are “continuing to tighten the noose” around Kyiv.
“This is definitely not over, they are still set on moving in, we’re reasonably confident. It’ll be utterly horrific when they do, I’m sure,” they said.
A second official warned of more repression of the people in Russia to come as the Russian president faces an extended campaign that he did not prepare his citizens or troops for.
“This will end because of a combination of factors, one of them is the impact of sanctions, but it’s only one of them,” they said.
“I think unfortunately the state’s response will not be to think about the best interests of ordinary Russians the state’s response will be to double down, to control information, to blame other people and try and see what they can do by the partnership they can do with China and a few other countries to offset the worst of it.”
They said that Russians are starting to notice Western measures such as restrictions on paying with credit cards and Apple Pay, as chains such as Starbucks and McDonalds close, but that the full impact of sanctions is yet to come.
They added: “At the moment it’s more inconvenience rather than hardship and it’s a shock for Russians who didn’t expect the invasion and are suddenly seeing the sorts of things that they’ve got used to that are normal in most European countries are suddenly being taken away.”