President Vladimir 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 am local time.

Explosions were heard soon after in the capital and other parts of Ukraine, reported news agencies and news television channels, prompting outrage from Joe Biden who warned of a "catastrophic loss of life".

• Ukraine cuts diplomatic ties with Russia
• Russia's ground forces on Thursday crossed into Ukraine from several directions, Ukraine's border guard service said, ndtv.com. reported quoting AFP.
• Russian tanks and other heavy equipment crossed the frontier in several northern regions, as well as from the Kremlin-annexed peninsula of Crimea in the south.
• Russian military says it has knocked out Ukraine's air defence assets and airbases, reports AP. PTI

Weeks of intense diplomacy and the imposition of Western sanctions failed to deter Putin, who had massed between 150,000 and 200,000 troops along the borders of Ukraine.

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.

He 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.

“I have taken the decision to carry out a special military operation,” 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. Our plans do not include occupying Ukrainian territory.”

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,” Putin said.

Russia alone responsible

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

“President Putin has chosen a premeditated war that will bring a catastrophic loss of life and human suffering,” 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.”

As Putin spoke, the U.N. Security Council met in a last-ditch attempt to avert war. Several ambassadors among the NATO allies, including the U.S. representative, Linda Thomas-Greenfield, called on Russia to pull back and return to the negotiating table.

“Back away from the brink before it is too late,” Thomas-Greenfield said.

India calls for restraint

At the United Nations, India has called for an immediate de-escalation of tensions between Russia and Ukraine and cautioned that the situation was in danger of spiraling into a major crisis.,

The 15-nation UN Security Council held an emergency meeting on Ukraine late Wednesday night, the second meeting this week and the fourth since January 31 as tensions escalated between Russia and Ukraine.

As the UNSC meeting was underway, during which UN Secretary General Antonio Guterres made a direct appeal to Putin to stop "your troops from attacking Ukraine", the Russian leader announced a military operation in eastern Ukraine.

India's Permanent Representative to the UN Ambassador T S Tirumurti told the Council that New Delhi had only two days ago at the Council meeting on Ukraine called for urgent de-escalation of tensions and emphasised on sustained and focused diplomacy to address all issues concerning the situation.

Inputs from The New York Times, PTI