KYIV, Ukraine (AP) — In a dramatic escalation of East-West tensions over Russia’s invasion of Ukraine, President Vladimir Putin ordered Russian nuclear forces put on high alert Sunday in response to what he called “aggressive statements” by leading NATO powers.

The order to put Russia’s nuclear weapons in an increased state of readiness for launch raised fears that the crisis could boil over into nuclear warfare, whether by design or miscalculation.

Amid the mounting tensions, Ukraine announced that a delegation would meet with Russian officials for talks. But the Kremlin’s ultimate intentions toward Ukraine — and what steps might be enough to satisfy Moscow — remained unclear.

The fast-moving developments came as Russian troops drew closer to Kyiv, a city of almost 3 million, street fighting broke out in Ukraine’s second-largest city, Kharkiv, and strategic ports in the country’s south came under pressure from the invading forces. Ukrainian defenders put up stiff resistance that appeared to slow the invasion.

Putin, in giving the nuclear alert directive, cited not only statements by NATO members but the hard-hitting financial sanctions imposed by the West against Russia, including the Russian leader himself.

Speaking at a meeting with his top officials, Putin told his defense minister and the chief of the military’s General Staff to put nuclear forces in a “special regime of combat duty.”

“Western countries aren’t only taking unfriendly actions against our country in the economic sphere, but top officials from leading NATO members made aggressive statements regarding our country,” Putin said in televised comments.

White House press secretary Jen Psaki said Putin is resorting to a pattern he used in the weeks before launching the invasion, “which is to manufacture threats that don’t exist in order to justify further aggression.” She told ABC’s “This Week” that Russia has not been under threat from NATO or Ukraine.

“And we’re going to stand up,” Psaki said, adding: “We have the ability to defend ourselves, but we also need to call out what we’re seeing here.”

NATO Secretary-General Jens Stoltenberg told CNN, in reaction to Putin’s decision to put Russia’s nuclear forces on high alert: “This is dangerous rhetoric. This is a behavior which is irresponsible.”

The practical meaning of Putin’s order was not immediately clear. Russia and the United States typically have land- and submarine-based nuclear forces on alert and prepared for combat at all times, but nuclear-capable bombers and other aircraft are not.

If Putin is arming or otherwise raising the nuclear combat readiness of his bombers, or if he is ordering more ballistic missile submarines to sea, then the United States might feel compelled to respond in kind, said Hans Kristensen, a nuclear analyst at the Federation of American Scientists. That would mark a worrisome escalation, he said.