Russia-Ukraine updates: Putin suspends key US-Russia nuclear treaty

Russian President Vladimir Putin declared a Christmas "truce" for the next 36 hours with a temporary cease-fire set to end at midnight on Jan. 7.

Air raid sirens sounded across several southern and eastern regions of Ukraine a few hours before the cease-fire started.

However, Ukrainian President Volodymyr Zelenskyy last night called the truce a "cover" to stop Ukraine from advancing in the east.

The Russian Orthodox Church celebrates Christmas Day on Jan. 7, but the brief unilateral Russian cease-fire called by Putin is not expected to last beyond the announced period.

The temporary cease-fire could put Ukraine in a difficult position. If it seeks to stop Russia from regrouping its forces during the cease-fire, it could be accused of violating a truce it never agreed to in the first place and be portrayed as aggressive. However, if Ukraine stops fighting for the 36-hour period, it will likely face a more dangerous position when the cease-fire ends.

Germany plans to provide Ukraine with a Patriot air defense battery, the White House confirmed Thursday.

The United States agreed last month to send the first Patriot missile defense system to Ukraine to protect the country against Russia’s ongoing missile and drone attacks targeting its critical infrastructure.

The development came as President Joe Biden spoke by phone Thursday with German Chancellor Olaf Scholtz about the ongoing war.

The White House also said it would send Bradley infantry fighting vehicles to Ukraine. Germany agreed to send Marder infantry fighting vehicles, according to the White House.

"They (Germany) reiterated their support for Ukraine’s sovereignty and independence," the White House said in a statement. "They reaffirmed their unwavering solidarity with Ukraine and the Ukrainian people in the face of Russia’s aggression."

-ABC News' Matt Seyler and Ben Gittleson

President Joe Biden said Thursday that he is skeptical of Russian President Valdimir Putin's proposal for a cease-fire during the Orthodox Christmas holiday, suggesting it was a ploy to find some breathing room in the war with Ukraine.

"I'm reluctant to respond to anything Putin says," Biden said.

Biden made the comments in response to a reporter's question following unrelated remarks at the White House about the U.S.-Mexico border.

"I found it interesting. He was ready to bomb hospitals and nurseries and churches," said Biden, referring to Russian missile strikes on Ukraine on Dec. 25 and over the New Year's Day holiday. "I think he's trying to find some oxygen."

-ABC News' Ben Gittleson

Russian President Valdimir Putin has called for a temporary cease-fire in the war with Ukraine to observe Orthodox Christmas, according to the Kremlin.

Putin proposed a cease-fire beginning at noon on Friday and ending at midnight Saturday, according to the Kremlin. Orthodox Christmas Day is on Saturday.

Putin instructed his minister of defense, Sergei Shoigu, to introduce the ceasefire along the entire line of contact between the warring countries in Ukraine during the holiday.

The Kremlin said the truce is being called for with "Christian love, true faith and crystal truthfulness."

There was no immediate response from Ukraine on whether its forces would abide by the truce.