By Jessie Yeung, Adam Renton, Rob Picheta, Ed Upright, Aditi Sangal, Adrienne Vogt, Melissa Macaya and Maureen Chowdhury, CNN
From CNN's Tim Lister in Kyiv
The General Staff of the Ukrainian Armed Forces has claimed five Russian aircraft and a helicopter were shot down early Thursday, as Russian forces attacked Ukraine.
The Russian military has denied the claims, state news agency TASS reported on Thursday.
From CNN's Zachary Cohen and Lauren Fox
US senators have said the unfolding attack in Ukraine is in line with intelligence briefings they received about what to expect from a Russian invasion.
In a series of tweets, Sen. Marco Rubio, the top Republican on the Senate Intelligence Committee, said that Russia was launching a “full scale and comprehensive military assault throughout Ukraine.”
The attack involved “airborne and amphibious landings, missile strikes from air, ground and naval forces, electronic and cyber attacks and a large ground force to occupy a large swarth of territory," Rubio said.
He added that Russian airborne forces are also working to “take control of the airport in Kyiv (so) they can fly in forces to occupy the (capital) city." A source familiar with the matter said the tweets were based on US intelligence being shared with Intelligence Committee members.
Congress briefed on attack: As the Russian attack escalated late Wednesday, Sen. Mark Warner, the Senate Intelligence chairman, and Rep. Adam Schiff, the chair of the House Intelligence Committee, spoke with CIA Director William Burns, according to two sources familiar with the matter.
Sen. Ben Cardin, a member of the Senate Foreign Relations Committee, told CNN that senators would be briefed on the invasion by the White House Thursday.
“A lot of what we’re seeing happening in regards to the apparent air attacks on the defense infrastructure of Ukraine is all part of what was expected we would see,” Cardin said. “There is no justification for it. I can tell you there’ll be strong bipartisan support in the United States Senate and Congress for the strongest possible reaction by the United States and our allies.”
From CNN's Nathan Hodge in Moscow
Several Russian airports have restricted operations in relation to the ongoing situation in Ukraine, according to a statement from Russia's Federal Air Transport Agency, Rosaviatsia.
“In contraction with the difficult situation around Ukraine, from the 24th of February at 3.45am Moscow time, flights in a number of airports in the South of Russia have been suspended," the statement read.
The restrictions will be in effect until March 2, according to the aviation authorities.
Flights are suspended at airports in:
"The aviation authorities of Russia strongly recommend that passengers on upcoming flights and those who are meeting flights go to those airports," the statement read. "All changes in the schedule of airports and airlines should be clarified on the official websites of airports and airlines, including social networks."
The mayor of Kharkiv, the second-largest city in Ukraine, has urged citizens to stay home as explosions and sounds of artillery are reported across the country.
"Please do not leave your homes today. Due to the complicated situation, schools, kindergartens and other institutions do not work today — until the situation becomes clearer," Mayor Igor Terekhov said in a Facebook post on Thursday morning.
"At the same time, public transport works as usual. I will keep you informed of what is happening and the actions of city authorities throughout the day.”
Elsewhere in Ukraine: The national capital Kyiv and the western city of Lviv have both sounded air raid sirens — but the mood is still "remarkably calm" on the ground, according to CNN reporters on the ground. It's about 8 a.m. there, with what looks like normal morning traffic as people head to work.
This post has been updated to more accurately describe Lviv.
From CNN's Katie Bo Lillis
The US Department of Defense is tracking the reported incursion of troops from Belarus into Ukraine, a US defense official said Thursday.
It was not clear if the troops were only Russian or also Belarusian, the source said.
Some context: Through a livestream video, CNN witnessed troops atop a column of military vehicles entering Ukraine from a border crossing with Belarus at about 6:48 a.m. local time Thursday.
The livestream video was taken at the Senkivka, Ukraine, crossing with Veselovka, Belarus.
In recent weeks, Russia has amassed a significant number of troops, vehicles and tanks in Belarus near the border with Ukraine. During that time, the two countries have held joint military exercises across the countries, and near the Belarus-Ukrainian border.
Early Thursday, the Ukrainian state border service said the country was attacked through multiple borders, including Belarus.
CNN reporters in Lviv, a city in western Ukraine, heard sustained air raid sirens just before 7:45 a.m. local time (12:45 a.m. ET).
Reporters in the country's capital of Kyiv also heard air raid sirens earlier Thursday morning.
Earlier this week, some diplomats relocated from Kyiv to Lviv as fears grew that a Russian attack on Ukraine would include the capital.
From CNN's Nathan Hodge in Moscow
The Moscow Exchange announced Thursday it had suspended trading on all of its markets until further notice.
From CNN’s Yoonjung Seo in Seoul
The South Korean embassy in Kyiv strongly urged its citizens in Ukraine to be ready to evacuate to a safe area in case of an emergency, in a notice published before Russian President Vladimir Putin announced the military operation in Ukraine.
“Our embassy learned on February 23 that all Russian embassy and consulate staff in Ukraine hastily left Ukraine, and some national embassy staff also left Kyiv,” said the embassy on its website early Thursday.
“Considering the urgent change in the current situation, we strongly recommend that the South Koreans who are still in Ukraine be fully prepared so that they can move to a safe area or location promptly in case of an emergency,” the embassy said.
The embassy also urged all South Korean nationals in Ukraine to maintain contact with the embassy.
From CNN's Laura He in Hong Kong
Asian markets and US stock futures plunged on Thursday as Russian President Vladimir Putin announced a military operation in Ukraine.
Hong Kong's Hang Seng Index declined 3.2%. Korea's Kospi dropped 2.7%. Japan's Nikkei 225 lost 2.4% after coming back from a holiday. China's Shanghai Composite moved 0.9% lower.
US stocks futures also tumbled. Dow futures were down as much as 780 points, or 2.4%. S&P 500 and Nasdaq futures were down 2.3% and 2.8%, respectively.
The broad losses followed a sharp decline on Wall Street on Wednesday. The Dow closed down more than 464 points, or 1.4%, posting its fifth straight day of losses. The S&P 500 and Nasdaq fell 1.8% and 2.6%, respectively.
Read more here.