Jake Sullivan, the US national security advisor, said the Joe Biden administration is worried about the possibility of Russian President Vladimir Putin’s nuclear escalation. However, Sullivan said: “we have not seen anything that would require us to change our nuclear posture at this time.”

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“We are watching this extremely closely, and obviously, the escalation risk with a nuclear power is severe, and it is a different kind of conflict than other conflicts the American people have seen over the years,” Sullivan was quoted as saying by CNN.

“And American President, Joe Biden, has to take that responsibility extremely seriously, even as we redouble our efforts to support the Ukrainians. As things stand today, the United States has not adjusted our nuclear posture, but it is something that we monitor day by day, hour by hour, because it is a paramount priority to the President,” he added.

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He also said that Russian strikes on Lviv, approximately 12 miles from Ukraine’s border with Poland, is a sign that Putin “is frustrated by the fact that his forces are not making the kind of progress that he thought that they would make against major cities, including Kyiv, that he is expanding the number of targets, that he is lashing out, and that he is trying to cause damage in every part of the country.”

On Sunday, waves of Russian missiles pounded a military training base near Ukraine’s western border with NATO member Poland, killing 35 people, Ukrainian authorities said Sunday. The strike followed Russian threats to target foreign weapon shipments that are helping Ukrainian fighters defend their country against Russia’s grinding assault.

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More than 30 Russian cruise missiles targeted the sprawling facility that is less than 25 kilometres (15 miles) from the closest border point with Poland, according to the governor of Ukraine’s western Lviv region. Poland is a transit route for Western military aid to Ukraine, and the United States increased the number of American troops deployed there.

The training base near Yavoriv appears to be the westernmost target struck during Russia’s 18-day invasion. The facility, also known as the International Peacekeeping and Security Center, has long been used to train Ukrainian military personnel, often with instructors from the U.S. and other NATO countries.