The possibility of Russia using chemical or biological weapons is deeply troubling the US and its allies.

After the Nato summit in Brussels – where the topic was discussed behind closed doors – President Biden said the use of chemical weapons by Russia would trigger a response "in kind".

He said "the nature of the response would depend on the nature of the use".

So what might it look like?

Biden's National Security Advisor Jake Sullivan spoke to reporters on Air Force One as the president flew to Poland.

He said: "We will select the form and nature of our response based on the nature of the action Russia takes and will do so in coordination with our allies."

But he also made clear: "The United States has no intention of using chemical weapons under any circumstances."

The US and Nato are being deliberately ambiguous about how they would respond if Vladimir Putin escalates his attack on Ukraine by using chemical weapons.

They'll only say that Russia will pay a severe price if it uses any kind of weapons of mass destruction - and that they have communicated that directly to Kremlin.

The White House has already assembled a national security task force – called the Tiger Team - to look at how it should react. And Nato is issuing protective equipment to its forces stationed in Eastern Europe.