(NEXSTAR) – Actors Ryan Reynolds and Blake Lively have pledged to match every donation to the United Nations Refugee Agency, up to $1 million, as thousands of Ukrainians are being forced from their homes, the couple tweeted Saturday.

Amid Russia’s invasion of Ukraine, the UN Refugee Agency says 368,000 have been displaced, and that number “continues to rise.”

Reynolds tweeted Saturday, saying in part, “In 48 hours, countless Ukrainians were forced to flee their homes to neighboring countries. They need protection.” Lively posted a similar message to Instagram.

While Europe has pledged to aid those fleeing Russia’s invasion the mass exodus of Ukrainians may overwhelm resources in neighboring countries, The Hill reports. As many as 5 million Ukrainians could be displaced by Russian forces, according to U.S. intelligence agencies.

Poland, Romania, the Czech Republic, and Slovakia are among those opening to refugees. Experts say some are expected to head to the U.S. as well.

President Vladimir Putin ordered Russian nuclear forces put on high alert Sunday in response to what he called “aggressive statements” by leading NATO powers.

The order means Putin wants Russia’s nuclear weapons prepared for increased readiness to launch and raises the threat that Moscow’s invasion of Ukraine and the West’s response to it could boil over into nuclear warfare.

Amid the worrying development, the office of Ukraine’s president said a delegation would meet with Russian officials as Moscow’s troops drew closer to Kyiv.

Earlier Sunday, the Ukrainian capital, Kyiv, was eerily quiet after huge explosions lit up the morning sky and authorities reported blasts at one of the airports. Only an occasional car appeared on a deserted main boulevard as a strict 39-hour curfew kept people off the streets. Terrified residents instead hunkered down in homes, underground garages and subway stations in anticipation of a full-scale Russian assault.

Until Sunday, Russia’s troops had remained on the outskirts of Kharkiv, a city of 1.4 million about 12.4 miles south of the border with Russia, while other forces rolled past to press the offensive deeper into Ukraine.

Videos posted on Ukrainian media and social networks showed Russian vehicles moving across Kharkiv and Russian troops roaming the city in small groups. One showed Ukrainian troops firing at the Russians and damaged Russian light utility vehicles abandoned nearby.

The images underscored the determined resistance Russian troops face while attempting to enter Ukraine’s bigger cities. Ukrainians have volunteered en masse to help defend the capital, Kyiv, and other cities, taking guns distributed by authorities and preparing firebombs to fight Russian forces.

The Associated Press contributed to this report.