Russia suddenly says it's pulling back troops from Ukraine's borders after weeks of tensions and fears of an invasion

• Russia abruptly announced it's pulling back troops from Ukraine's borders after weeks of tensions.
• This came one day after Putin warned the West against crossing Russia's "red line."
• The buildup of roughly 100,000 Russian troops along Ukraine's borders sparked fears of an invasion.

Russia on Thursday abruptly announced it's pulling troops back from Ukraine's borders, just one day after President Vladimir Putin warned that anyone who threatens Russia's security will "regret" it.

The buildup of roughly 100,000 Russian troops over the past few weeks along the eastern Ukrainian border and in Crimea, which Putin unilaterally annexed in 2014, raised alarms across the West and sparked fears of an invasion.

But Russian Defence Minister Sergei Shoigu on Thursday said that the exercise had achieved its goal and ordered the troops to withdraw from the area to their permanent bases by May 1. "The troops have demonstrated their ability to provide a credible defence for the country," Shoigu said, per BBC News.

The order also specified, however, that the troops in one field camp roughly 100 miles from the eastern Ukrainian border leave their armored vehicles there until fall, the New York Times reported.

Ukrainian President Volodymyr Zelensky cautiously welcomed the development.

"The reduction of troops on our border proportionally reduces tension," Zelensky said in a tweet. He added that Ukraine is "always vigilant" but "welcomes any steps to decrease the military presence" and "deescalate the situation" in Donbass - the war-torn eastern region where Ukraine has been fighting a war against Kremlin-backed rebels since 2014. More than 13,000 people have been killed as a result of the conflict.

Russia's massive troop buildup in recent weeks came amid heightened tensions with the US and NATO. As Western countries raised concerns about the tens of thousands of Russian troops amassing on Ukraine's borders, Russia said the troop buildup was part of readiness drills in response to NATO threats.

Military leaders and analysts generally interpreted the buildup as saber-rattling, or an attempt by Russia to display what it's capable of militarily. Last week, the top US general in Europe said there was a "low to medium" risk of a Russian invasion in Ukraine, but added that the likelihood of such an incursion would "start to wane" based on "the trend that I see right now."