President Joe Biden is ruling out sending U.S. ground troops to Ukraine if Russia decides to invade the neighboring nation for the second time since 2014, saying the option is “not on the table.”

Biden said he will impose “severe economic consequences like none he’s ever seen,” and that an invasion would lead to an increased NATO presence in the region.

Biden spoke with Russian leader Vladamir Putin on Tuesday during a video call and said that Putin “understood” the consequences of a Russian invasion. Putin made it clear that he does not want Ukraine to join NATO and that he does not want the U.S. to encroach on Russia’s borders.

Biden said the U.S. has a “moral and legal obligation to our NATO allies” to respond if they are attacked. According to The Wall Street Journal, Putin told Russian broadcast station TV5 Biden was “open, substantive and constructive,” adding “there is no particular reason for optimism so far.” 

The U.S. has been training Ukrainian troops since 2015 and U.S. officials don’t know if Putin has made up his mind whether to attack Ukraine or not.