After Russian President Vladimir Putin declared that he wants to redraw Europe’s borders by dismembering Ukraine, Western officials said Monday that they believe Russia is violating international law and are planning a strong response.The European Union and its partners “will react with unity, firmness and with determination in solidarity with Ukraine,” European Commission Chief Ursula von der Leyen tweeted, accusing Moscow of derailing negotiations between Ukraine’s government in Kiev and pro-Russian separatists in the east of the country. Putin said Monday that the separatist-controlled regions should now be treated as independent of Ukraine. He is widely expected to send Russian forces into the area ― known as the Donbas ― in the coming days and has amassed tens of thousands troops near other parts of Ukraine. In a statement, White House press secretary Jen Psaki said the Biden administration will bar Americans from supporting the separatists and sanction people who do. She noted that the U.S. is separately considering major sanctions on Russia if it ramps up its interference in Ukraine, where it has been supporting the insurgency since 2014. British Foreign Secretary Liz Truss said her nation would unveil new sanctions on Tuesday.National security experts worry that intense fighting will soon break out, and Western intelligence suggests Putin will claim that the Ukrainians have initiated it.Alar Karis, the president of Estonia, tweeted, “Moscow is not serious about diplomacy but is looking for casus belli,” referring to a justification for war.“Moscow continues to fuel the conflict in eastern Ukraine by providing financial and military support to the separatists. It is also trying to stage a pretext to invade Ukraine once again,” NATO Secretary General Jens Stoltenberg said Monday. The Donbas conflict has already killed more than 13,000 people.Ukraine is not a U.S. treaty ally, and neither American nor European forces are expected to engage in any fighting with Russians or their partners. But Kiev has received significant Western military support, and President Joe Biden has deployed U.S. forces to neighboring allied countries like Poland and Romania to signal unity against Putin’s bid for greater regional influence. Russia has urged Washington to withdraw military support from those states ― despite their independent status and longstanding membership in the NATO alliance.Ukrainian President Volodymyr Zelenskyy spoke with Biden on Monday and tweeted that he is gathering his top national security aides and reaching out to other global leaders, including British Prime Minister Boris Johnson.Nina Golgowski contributed reporting.