Russia-Ukraine updates: US to ban Russian carriers from its airspace

Ukrainian President Volodymyr Zelenskyy declared martial law in the country Thursday, saying Russia has launched an "unjustified, false and cynical invasion."

"There are strikes on military and other important defense facilities, attacked border units, the situation in the Donbas has degraded," Zelenskyy said in a statement Thursday morning. "The Armed Forces, all special and law enforcement agencies of the state are on alert. The National Security and Defense Council is working in an emergency mode."

"Civilian citizens of Ukraine should stay at home," he added. "Warn your loved ones about what is happening. Take care of those who need help. All thoughts and prayers with our soldiers."

Martial law allows military authorities to temporarily take over government functions, generally during a time of emergency.

The announcement came as reports of explosions and air raid sirens in cities across Ukraine rolled in and as Russian-controlled separatists, in a breakaway region of eastern Ukraine known as Donbas, say they have launched a full-scale offensive to retake what they claim is their territory there.

In a new video statement denouncing the Russian attacks on his country, Ukrainian President Volodymyr Zelenskyy said it’s not only the fate of Ukraine that’s being decided.

“Ukrainians will never give their freedom and independence to anyone. Only we, all citizens of Ukraine, have been determining our future since 1991,” Zelenskyy said. “But now the fate of not only our state is being decided, but also what life in Europe will be like.”

The Ukrainian president stressed the need for a global response, stating that what remains of international law “depends on the world’s honest and just response to this aggression.”

President Joe Biden and Zelenskyy spoke over the phone around midnight ET, when Zelenskyy asked Biden to “call on the leaders of the world to speak out clearly against President Putin’s flagrant aggression and to stand with the people of Ukraine,” Biden said in a statement.

Biden said he told Zelenskyy that the U.S. condemned the attack, and he also briefed him on the steps the U.S. is taking “to rally international condemnation.”

Biden also reiterated in the statement that he will meet with G-7 leaders Thursday and plans to impose “severe” sanctions on Russia.

“We will continue to provide support and assistance to Ukraine and the Ukrainian people,” he said.

In the video, Zelenskyy implored citizens to stay home.

“Warn your loved ones about what is happening,” he said. “Take care of those who need help.”

Senators on both sides of the aisle are calling on the administration to turn up the pressure on Russia following its attack on Ukraine.

"President Biden has already imposed an initial tranche of sanctions, and it is now time for us to up the pain level for the Russian government," Senate Intelligence Committee Chairman Mark Warner, D-Va., said in a statement.

"I urge the Biden administration to respond swiftly and in concert with our allies to impose crushing economic sanctions on Kremlin officials, Russian entities and other actors involved int his attack on Ukraine," Sen. Jeanne Shaheen, D-N.H., said.

"America and our allies must answer the call to protect freedom by subjecting Putin and Russia to the harshest economic penalties, by expelling them from global institutions, and by committing ourselves to the expansion and modernization of our national defense," Sen. Mitt Romney, R-Utah, said.

While a handful of Republicans knocked the administration for not imposing pre-invasion sanctions, those jabs are largely being overwhelmed by calls for unity among NATO allies.

"There is no justification for this assault and I call for the administration to lead the world in a unified response," Ohio Republican Sen. Rob Portman said.

-ABC News' Allie Pecorin

There are reports that Russian troops have now crossed the border in eastern Ukraine, close to the city of Kharkiv, as explosions in the area continue.

Ukraine’s deputy interior minister, Anton Gerashchenko, told media the troops crossed near Kharkiv. A former senior adviser to Ukraine’s government also confirmed the reports, saying he was informed by the president’s office, while Ukraine’s main newswire agency, UNIAN, has also reported the news. 

Kharkiv is Ukraine’s second largest city and is only about 20 miles from the border with Russia. However, it is not next to the separatist-controlled areas.

Ukrainian authorities also said that Kyiv and multiple cities east of it have been stuck by ballistic missiles. Gerashchenko said at least seven cruise missiles or ballistic missiles hit a military aerodrome near Kyiv that is home to fighter jets. It appears that the missiles have largely struck targets on the outskirts of the city so far.

Russia’s defense ministry said it is striking Ukrainian air bases, military infrastructure and air defenses across the country but said it will not target Ukrainian cities themselves.

The ministry said “high-precision” missiles are being used.

Reports of explosions also continue to come in from Odessa, Dnipro and Mariupol.

-ABC News’ Patrick Reevell