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

Amid Russia’s invasion of Ukraine, says UN Refugee Agency 368,000 have been displacedand this number “continues to increase”.

Reynolds tweeted Saturday, stating in particular: “In 48 hours, countless Ukrainians have been forced to flee their homes to neighboring countries. They need protection. Lively posted a similar message to instagram.

While Europe has pledged to help those fleeing the Russian invasion, the mass exodus of Ukrainians could overwhelm the resources of neighboring countries, The Hill Reports. According to US intelligence agencies, up to 5 million Ukrainians could be displaced by Russian forces.

Poland, Romania, the Czech Republic and Slovakia are among those opening to refugees. Experts say some should also travel to the United States.

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

The order means Putin wants Russia’s nuclear weapons ready to go and raises the threat that Moscow’s invasion of Ukraine and the West’s response to it could escalate into nuclear war.

Amid the worrying development, the Ukrainian president’s office said a delegation would meet with Russian officials as troops from Moscow closed in on Kiev.

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

Until Sunday, Russian troops had remained on the outskirts of Kharkiv, a city of 1.4 million people about 20 km south of the border with Russia, while other forces moved in to ram the offensive deeper into Ukraine.

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

The footage highlighted the determined resistance Russian troops face when trying to enter major Ukrainian cities. Ukrainians volunteered en masse to help defend the capital, Kyiv, and other cities, taking weapons distributed by authorities and preparing firebombs to fight Russian forces.

The Associated Press contributed to this report.