Nato chief Jens Stoltenberg commented on the situation following Moscow's repeated allegations that Ukraine could use such a weapon, sparking fears Russia could use one and blame Kyiv.
Mr Stoltenberg said he had spoken with Pentagon chief Lloyd Austin and Britain's Defence Secretary Ben Wallace “about Russia's false claim that Ukraine is preparing to use a dirty bomb on its own territory”.
“Nato allies reject this allegation. Russia must not use it as a pretext for escalation. We remain steadfast in our support for Ukraine,” he wrote on Twitter.
Moscow has alleged that Ukraine is close to developing a dirty bomb, and Foreign Minister Sergey Lavrov insisted the threat is real.
“This is not empty information … there are serious suspicions that such things may be planned,” Mr Lavrov said. “We have a keen interest in preventing such a terrible provocation.”
But State Department spokesman Ned Price said Washington is worried that Russia's claims could be a cover.
“We have seen a pattern in this conflict and the lead-up to this war where the Russians have engaged in mirror imaging — the Russians have accused the Ukrainians, the Russians have accused other countries of what itself was planning. That is our concern”, Mr Price said.
The head of the Russian army Valery Gerasimov repeated Moscow's claims in a telephone call with his US counterpart on Monday, the defence ministry said.
Ukraine war latest — in pictures
The call was the latest in a string of conversations between Russian defence officials and counterparts from Nato countries, during which Moscow said, without providing evidence, that Kyiv was planning to deploy a dirty bomb.
“According to the information we have, two organisations in Ukraine have specific instructions to create a so-called dirty bomb. This work is in its final stage,” said Russian Lt Gen Igor Kirillov in a statement on Monday.
At its most basic, a dirty bomb is a conventional weapon laced with radioactive, biological or chemical materials that are disseminated in an explosion.
Moscow's claims follow weeks of military defeats for Russia in southern and eastern Ukraine, with observers and Kyiv saying the Kremlin is becoming increasingly desperate.
In Kyiv's latest announcement of territorial gains, the Ukrainian military claims to have pushed Russian forces from several villages in the north-east of the country.
“Due to successful actions, our troops pushed the enemy out of the settlements of Karmazynivka, Myasozharivka and Nevske in the Lugansk region and Novosadove in Donetsk region,” the Ukrainian military said in a statement.
Both Kyiv and its allies have fiercely rejected the dirty bomb claims, which follow thinly veiled threats from Moscow of potential nuclear escalation.
Ukrainian Foreign Minister Dmytro Kuleba said the head of the UN-run International Atomic Energy Agency, Rafael Grossi, accepted his request to “urgently send experts to peaceful facilities in Ukraine, which Russia deceitfully claims to be developing a dirty bomb”.
Putin vows continued 'tough' attacks against Ukraine — video
The IAEA confirmed in a statement that it would visit two Ukrainian facilities “in the coming days”.
“Our countries made clear that we all reject Russia's transparently false allegations that Ukraine is preparing to use a dirty bomb on its own territory,” the statement said.
With the help of western-supplied heavy weapons, Ukraine has managed to claw back parts of its territory from Russia in the east and south, while its power grid has been pummelled before winter.
As momentum has swung towards Ukraine, Russian President Vladimir Putin has faced fissures in domestic support for his campaign, as a messy troop draft and battlefield losses challenged the prospect of a quick conclusion.
The Kremlin meanwhile said on Monday that France and Germany were showing “no desire” to participate in mediation of the conflict and praised Turkish President Recep Tayyip Erdogan's offer to organise talks.
Turkey helped broker the deal that allowed grain exports to resume under the UN's aegis in July, and played a role in a prisoner swap in September, one of the largest exchanges.