President Biden told Russian President Vladimir Putin in a phone call on Thursday that the U.S. and its allies will react "decisively" if Ukraine is invaded, White House press secretary Jen Psaki said. A senior administration official said Mr. Biden and Putin and agreed to a "sequence of strategic stability dialogue." The call comes as Russia has built up troop levels along the border with Ukraine, and U.S. intelligence has warned Russia could invade Ukraine as soon as January. Meanwhile, a U.S. official confirmed to CBS News that an American spy plane flew a mission over Ukraine on Thursday, which followed another flight on December 27. The flight was by a JSTARS aircraft, which is a radar plane specifically designed to track ground movements. In a statement, the U.S. European Command said it routinely operates aircraft in the Black Sea region in support of "various U.S. and coalition intelligence objectives." The U.S. European Command said the event was preplanned and "not in response to any specific event." The senior administration official said Mr. Biden laid out two paths, one focused on democracy and the other focused on deterrence and "significant costs and consequences should Russia choose to proceed with further invasion of Ukraine." The senior administration official said the tone of Mr. Biden's call with Putin was "serious and substantive" and its primary purpose was to set the "tone and tenor of diplomatic engagement" scheduled for next month. The upcoming talks will help "determine more precisely the contours" of the areas where there can be agreement between the two leaders, the official said. Mr. Biden urged Putin to deescalate tensions with Ukraine, and the official said the U.S.' focus is "really on actions and on indicators not on words" and will continue to prepare for whatever decision Putin makes with regard to Ukraine. The U.S. will continue a "very intensive period of consultation" with allies over the next week before the NATO-Russia council talks, the senior administration official said. "There will be a very careful and very intensive coordination transparency among our partners and allies," the official said. Russia has built up an estimated 100,000 troops near the Ukrainian border. Despite recent reports that the Russians have withdrawn 10,000 troops from the area, the senior administration official said the White House is "greatly concerned" about the still "significant Russian troop presence" along the border. CBS News has reported that the 10,000 withdrawn troops is just a fraction of the number that still remains at the border. David Martin, Fin Gomez and Fritz Farrow contributed to this report. 