BRUSSELS (Reuters) -- Western nations will warn Russian President Vladimir Putin on Thursday that his country will pay "ruinous" costs for invading Ukraine, during an unprecedented one-day trio of NATO, G-7 and EU summits that will be attended by U.S. President Joe Biden.
The hectic day of summitry to maintain Western unity will kick off at NATO headquarters in Brussels, where the trans-Atlantic defense alliance's leaders will agree to ramp up military forces on Europe's eastern flank.
Alarmed by the prospect that Russia might escalate the war with its neighbor after a grinding monthlong conflict, the 30 nations of NATO will also agree to send Kyiv equipment to defend against biological, chemical and nuclear attacks.
The resolve to punish Moscow with massive sanctions will be underlined by an emergency meeting of the G-7 advanced economies.
Then, with a summit of the 27-nation European Union, countries representing more than half the world's gross domestic product will have met in one day.
"We must ensure that the decision to invade a sovereign independent country is understood to be a strategic failure that carries with it ruinous costs for Putin and Russia," Canadian Prime Minister Justin Trudeau told the EU parliament.
Russia's assault on Ukraine has killed thousands and driven almost a quarter of Ukraine's 44 million people from their homes, according to United Nations data, including more than 3.6 million who have fled the country.
Putin says his forces are engaged in a "special military operation" to demilitarize and "denazify" Ukraine. Ukraine and the West say Putin launched an unprovoked war of aggression.
"President Putin has made a big mistake," NATO Secretary-General Jens Stoltenberg said as he arrived at the meeting, pointing to the strength of Ukraine's resistance and the unity of the West.
In this "most serious security crisis in a generation," Stoltenberg said, "as long as we stand together, we are safe."
Ukrainian President Volodymyr Zelenskyy will address both the NATO and EU summits via videoconference.
He has pleaded, without success, for NATO to create a no-fly zone over Ukraine, but Western allies have imposed sweeping sanctions against Russia and provided weapons and aid worth billions of dollars for Ukraine's defense.
NATO has sharply increased its presence on its eastern borders, with some 40,000 troops spread from the Baltic to the Black Sea. NATO's Stoltenberg said the leaders would discuss deploying four new combat units in Bulgaria, Romania, Hungary and Slovakia.
"I expect leaders will agree to strengthen NATO's posture in all domains, with major increases in the eastern part of the alliance. On land, in the air and at sea," he said.
Washington said Biden and his European counterparts would announce new sanctions against Russia and measures to tighten existing sanctions. However, EU diplomats played down expectations of major new sanctions.
Russia has been frozen out of world commerce to a degree never before visited on such a large economy. But the biggest loophole is an exception for its energy exports. Some EU member states are resisting calls to ban Russian oil and gas, as they rely heavily on them.
EU leaders are expected to agree at their two-day summit to jointly buy gas, as they seek to cut reliance on Russian fuels and build a buffer against supply shocks.
Brussels is also aiming to strike a deal with Biden to secure additional U.S. liquefied natural gas supplies for the next two winters.
"The consequences of this war on Europe's security architecture will be far-reaching," the president of the EU's executive, Ursula von der Leyen, said on Wednesday. "And I am not just talking about security in military terms. But also energy security and even food security are at stake."