During a speech in Delaware, President Obama spoke about Malaysia Airlines Flight MH17, which an adviser to Ukraine's interior minister said that was flying over a town in eastern Ukraine when it was hit by a missile on Thursday. (WhiteHouse.gov)

This post has been updated

President Obama made his first remarks on the Malaysian plane that crashed in Ukraine Thursday, saying that the "world is watching" reports of the downed airliner.

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

“The United States will offer any assistance we can to help determine what happened, and why,” he said.

(Live Updates: Malaysia Airlines plane crashes in Ukraine)

Obama spoke with Russian President Vladimir Putin earlier on Thursday, fewer than 24 hours after the U.S. targeted Russia with expanded sanctions for its incursion into Crimea and just as reports of the crash were surfacing.

According to the White House, Obama "noted the early reports" of the downed Malaysian Airlines jet near the Russia-Ukraine border on the call. The plane was carrying 295 people and there are indications that it may have been shot down by an antiaircraft missile.

White House Press Secretary Josh Earnest said the call was made at the request of the Russian government. Earnest said Obama had been briefed on reports of the plane and directed his team to be in touch with senior Ukrainian officials.

After President Obama announced the toughest sanctions yet on Moscow, Russian President Vladi­mir Putin is warning the measures will "have a boomerang effect" against the U.S. (Reuters)

Obama told Putin that sanctions were not his "preferred course of action" and that he remains "committed to  a diplomatic solution," according to the White House. But Obama said that due to evidence that Russia is increasing the number of "heavy weapons" it is giving to Ukrainian separatists and failing to take steps outlined by the United States and other countries, it was necessary to impose sanctions.

Obama also told Putin he was concerned that about the continued buildup of Russian forces on the Ukrainian border and that Russia needs to, among other steps, push for a cease-fire and stop the flow of weapons into Ukraine.

Obama "noted that Russia would face continued costs and isolation unless it takes these concrete steps," the White House said.

The White House said Obama  and Putin agreed "on the need for a peaceful resolution to the Ukraine crisis achieved through diplomatic means."

Earnest reiterated to reporters aboard Air Force One, which Obama was taking to an event in Delaware, that the United States and other countries respect Ukraine's territorial integrity and that Russia can play a "constructive role" in de-escalating the conflict.

"Well, as you heard the president say before, his relationship with President Putin is very businesslike, that they do have a candid exchange of views when they have the opportunity to speak.  That occurred again today," Earnest said.