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President Joe Biden said on Thursday that the US “would respond” if Russia uses chemical weapons during its invasion of Ukraine.
“The nature of the response would depend on the nature of the use,” said Mr Biden in Brussels, where he is meeting Nato, G7 and European Union colleagues on a day of back-to-back diplomatic summits.
His comments came after Ukraine's President Volodymyr Zelenskyy said in an address to the G7 summit that the danger of Russia using chemical weapons was “quite real".
He said he had received information on Thursday that Russian troops had used phosphorus bombs against civilians in Ukraine.
Nato responded to concerns over chemical weapons use on Thursday by offering Ukraine medical supplies and decontamination training as part of a package of support.
This could also include detection and protection equipment and is intended to prepare Ukraine for the possibility of a chemical, biological, radiological or nuclear attack.
Biden says Nato or US 'would respond' if Putin uses chemical weapons
Western leaders have expressed fear that Russia's claims about Ukraine developing biological weapons — dismissed as untrue by the US and its allies — are a prelude to a false-flag operation.
UK Prime Minister Boris Johnson said any use of such weapons would be a “profound and a disastrous mistake” by the Russian leadership, which he described as having “crossed the red line into barbarism".
The G7 meeting meanwhile ended with a warning to Russia not to use chemical or biological weapons and a reminder of Moscow's obligations under international treaties.
The leaders said they “categorically denounce Russia’s malicious and completely unfounded disinformation campaign against Ukraine, a state in full compliance with international non-proliferation agreements".
Finland's Prime Minister Sanna Marin told reporters as she arrived at European Council headquarters that “Russia is capable of anything".
“They don’t respect any rules,” she said of the Russian leadership. “They don’t respect any international laws that they are actually committed to.”
The warnings came on a marathon day of diplomacy in which Nato resolved to strengthen its own defences as well as give equipment to Ukraine to help it prepare for chemical attacks.
Mr Biden said the US would answer Mr Zelenskyy's call with “significant and increasing amounts of security assistance” while Britain said it was sending 6,000 more defensive missiles to Ukraine as part of a new support package.
Within the bloc, Nato leaders approved the establishment of four new battalions in Hungary, Slovakia, Romania and Bulgaria to shore up the alliance's eastern flank.
