U.S. President Barack Obama was on the phone with Russia President Vladimir Putin on Thursday when he first learned about the Malaysia Airlines plane crash near the Russia-Ukraine border, the White House and Kremlin said this afternoon.

Putin had previously requested the call to discuss the ongoing crisis in eastern Ukraine, including the new round of economic sanctions that Obama levied against Moscow on Wednesday. Near the end of their conversation, Putin “informed the U.S. president of the report from air traffic controllers that the Malaysian plane had crashed on Ukrainian territory, which had arrived immediately before the phone call,” the Kremlin said in an online statement.

Obama later said the MH17 crash “looks like it might be a terrible tragedy.” Speaking in Delaware at a briefing on federal transportation funding, he said that the White House was “in close contact with the Ukrainian government” and that the U.S. “will offer any assistance to determine what happened and why. As a country, our thoughts and prayers are with all the passengers.”

He added that, “Right now, we’re working to determine whether there were American citizens on board. That is our first priority.” After Obama's speech, it was reported that at least 23 U.S. citizens were among the 295 victims aboard the flight from Amsterdam to Kuala Lumpur, but the U.S. State Department has yet to confirm that figure.

U.S. Vice President Joe Biden later called Ukrainian President Petro Poroshenko to offer help in the plane crash probe, a White House press secretary said.