Jack Reed, chair of the Senate armed services committee, said on Wednesday that an attack on a coalition member could be considered if Russia used a nuclear weapon on Ukraine.
According to US military news site Military.com, Reed said that if a nuclear device is detonated and the radiation goes into a neighboring country, it could be seen as an attack against NATO.
An attack on one NATO nation is an attack on all its members according to the alliance's defensive pact. If Putin ordered a chemical, biological, or nuclear strike on Ukraine, there would be consequences.
Reed said that the US is unlikely to take military action on its own if NATO decides to attack, according to the outlet.
He said that it would be a difficult call, but that the entire NATO Council would have to make it.
NATO Secretary-General Jens Stoltenberg said on the same day that the use of chemical weapons or biological weapons in Ukraine may have dire consequences for NATO countries and their peoples.
While NATO would try to help Ukraine find ways to shore up its defenses against a chemical attack, nearby member nations were still at risk of the spread of chemical or biological agents.
He said that it underscores the seriousness of the concerns.
On February 27, three days after the invasion of Ukraine began, Putin put his nuclear forces on high alert and blamed NATO for what he described as illegitimate sanctions and hostile actions.
On Monday, Biden warned that Moscow could be planning chemical or biological attacks now that Putin's back is against the wall.
Biden said that the Russian leader had already used chemical weapons and that the US should be careful.
If Russia deployed chemical weapons in Ukraine, Biden said Russia would pay a severe price.