Putin announces a ‘military operation’ in Ukraine as the U.N. Security Council pleads with him to pull back.

MOSCOW — President Vladimir V. Putin of Russia declared the start of a “special military operation” in Ukraine, pledging he would seek to demilitarize but not occupy the country, addressing the nation in a televised speech broadcast just before 6 a.m. local time.

Mr. Putin said the operation would aim for the “demilitarization and denazification of Ukraine,” referring to the Kremlin’s contention that Ukraine’s military threatens Russia and that it is run by neo-Nazis.

As he spoke, large explosions were heard and seen in Kyiv, Kharkiv, Odessa and other parts of Ukraine.Ukrainian officials said that rockets hit fighter jets at an airport near Kyiv and that Russian troops had crossed the border in Kharkiv and Odessa.

“The purpose of Russia’s offensive military operation is to destroy the Ukrainian state, seize Ukrainian territory by force, and establish occupation control,” Georgiy Tykhyi, a spokesman for Ukraine’s ministry of foreign affairs, said in a statement. “Russian troops are attacking peaceful Ukrainian cities from various directions.”

President Biden said in a statement the world would “hold Russia accountable” for what he cvalled a “unprovoked and unjustified attack.”

In his televised speech, Mr. Putin said he was acting after receiving a plea for assistance from the leaders of the Russian-backed separatist territories formed in eastern Ukraine in 2014. He said Russia does not intend to occupy Ukraine.

“I have taken the decision to carry out a special military operation,” Mr. Putin said. “Its goal will be to defend people who for eight years are suffering persecution and genocide by the Kyiv regime. For this we will aim for demilitarization and denazification of Ukraine, as well as taking to court those who carried out multiple bloody crimes against civilians, including citizens of the Russian Federation.”

Mr. Putin called on Ukrainian soldiers to immediately lay down their arms.

“All service members of the Ukrainian army who follow these demands will be able to leave the battle zone,” he said. He also offered what appeared to be a warning to other countries, such as the United States, that have supported Ukraine.

“Anyone who tries to interfere with us, or even more so, to create threats for our country and our people, must know that Russia’s response will be immediate and will lead you to such consequences as you have never before experienced in your history,” Mr. Putin said. “We are ready for any turn of events.”

Mr. Putin cast his operation both as an attack on “Nazis” in Ukraine, as well as rejection of the American-led world order. Ukraine’s aspiration to join NATO, he said, represented a dire threat to Russia. He evoked the NATO bombing of Yugoslavia in 1999 and the American invasion of Iraq in 2003 to make it clear that he viewed the West as morally bankrupt.

“For 30 years, we deliberately and patiently tried to reach agreement with the NATO countries on equal and indivisible security in Europe,” Mr. Putin said. But Russia was met, he said, with “cynical lies” and “blackmail” on the part of the West.

The American-led West, he said, represented an “empire of lies.”

In his statement, President Biden sought to place responsibility squarely on Mr. Putin’s shoulders.

“President Putin has chosen a premeditated war that will bring a catastrophic loss of life and human suffering,” Mr. Biden said. “Russia alone is responsible for the death and destruction this attack will bring, and the United States and its allies and partners will respond in a united and decisive way. The world will hold Russia accountable.”

In his speech, Mr. Putin went on to address the Russian people, arguing that they were on the right side of history.

“You and I know that our strength lies in fairness and truth, which is on our side,” Mr. Putin said. “And if this is so, then it is hard not to agree that it is strength and readiness to fight that are the foundation for independence and sovereignty.”

Michael D. Shear contributed reporting.