President Obama, in his first remarks about the downed passenger jet in Ukraine, pledged to offer the Malaysian airline U.S. assistance to determine what happened and why and said his administration's first priority is determining whether there were any Americans on board.

“It looks like this might be a terrible tragedy,” Obama said, adding that his team is trying to determine whether there were any Americans onboard the plane. “That is our first priority.”

“And I've directed our national security team to stay in close contact with the Ukrainian government. The United States will offer any assistance we can to help determine what happened and why, and as a country, our thoughts and prayers are with all the countries and passengers - wherever they call home,” Obama added.

Obama was in Delaware for remarks about the need for Congress to pass more infrastructure spending to fix bridges and roads. Several media outlets are reporting that 23 Americans were on board the passenger jet that crashed near the Russia-Ukraine border, but U.S. authorities have yet to confirm those reports.

Obama spoke to Russian President Vladimir Putin spoke by phone Thursday morning to discuss the new sanctions the U.S. imposed on Russia Wednesday, and near the end of the call Putin noted the early reports of a downed passenger jet near the Russia-Ukraine border.

White House press secretary Josh Earnest said that Moscow requested the call between the two leaders, which provided the opportunity for the Obama “to make clear once again the important principles at stake here” when it comes to Russia's support of rebel militias in Ukraine.

"The United States is committed to ensuring that this international norm of countries respecting the territorial integrity of other countries is prioritized,” Earnest said.

"The president made clear that the international community, the United States and our European allies are willing to take steps and impose economic costs on Russia if they decline to respect those basic norms,” he added.

Once again, Obama advised Putin that Russia could take steps to reduce the need for sanctions and if it did not, the U.S. would continue to increase the economic penalties on Moscow, Earnest said, describing the tone of the call as “candid” and “business-like.”

While Putin mentioned reports of a downed passenger plane toward the end of the call, Earnest said he could not confirm reports that pro-Russian separatists in Ukraine shot down a Malaysian passenger plane carrying more nearly 300 people.

“I’m not in a position to confirm” the plane has crashed, he said. “I can confirm that we’ve seen those reports, but I’m not in a position to confirm any details of those reports.”

Earnest said Obama has been briefed on the reports and has directed his team to be in close touch with senior Ukrainian officials on the matter.

A Ukrainian official told the Associated Press Thursday morning that a passenger plane carrying 295 people was shot down earlier in the day over a town in the east of the country. Malaysian Airlines tweeted that it lost contact with one of its flights over Ukrainian airspace.

The AP reported that the fate of the passengers has yet to be determined.

Ukrainian President Petro Poroshenko said his country's military was not responsible for the downed passenger plane.

"We do not exclude that this plane was shot down, and we stress that the Armed Forces of Ukraine did not take action against any airborne targets," he said. "We are sure that those who are guilty in this tragedy will be held responsible."

State Department spokeswoman Jen Psaki also said Thursday afternoon that she didn't have any "confirmed information about deaths or casualties or additional details" about the reports of the downed passenger plane.

"Our thoughts and prayers go out to the families of the victims involved," she said. "We're closely monitoring the situation and we're seeking additional information" while working with our embassy in Kiev.

Reporters asked Psaki whether 23 Americans were on board the downed plane and whether she could confirm reports that it was shot down by pro-Russian separatists with a Russian-made rocket.

"We don't have any additional details on American citizens. As soon as we have additional details, we will let you know," she said.