President Obama spoke with Ukrainian President Petro Poroshenko on Thursday to discuss the crash of a Malaysia Airlines plane in Ukraine that U.S. officials said was brought down by an antiaircraft missile.

Obama "assured him that U.S. experts will offer all possible assistance immediately," according to the White House, which said Poroshenko "welcomed the assistance."

The White House said both men said evidence from the crash must remain in the country until international investigators can ascertain what happened to the plane, which crashed in an area of eastern Ukraine near the Russian border. Officials said 295 people were aboard.

Vice President Biden also spoke with Poroshenko to extend an offer of American assistance.

Obama spoke with Secretary of State John Kerry about the plane crash and recent developments in Israel and the Gaza Strip. He was also briefed by senior members of his national security team, including CIA Director John Brennan, on the crash and efforts to support the Ukrainian government.

"The president directed his national security team to continue offering whatever assistance is necessary to advance the international effort to determine what happened," White House press secretary Josh Earnest said.

Earlier in the day, Obama briefly discussed the downed airliner during a stop in Wilmington, Del.

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

Obama spoke with Russian President Vladimir Putin Thursday morning, just as reports of the plane crash were filtering in. According to the White House, Obama "noted the early reports" of a downed Malaysia Airlines jet near the Russia-Ukraine border on the call. Obama and Putin discussed the expanded sanctions that the United States leveled against Russia on Wednesday for its support of separatists in Ukraine.

Obama also called Malaysian Prime Minister Najib Razak on Thursday to express his condolences and offer “immediate assistance to support a prompt international investigation,” according to the White House. Obama offered any assistance or support the Malaysian government may need.

Biden also addressed the crash during a speech in Detroit at Netroots Nation, an annual progressive gathering.

Biden said the plane was “apparently” shot down – “shot down, not an accident, blown out of the sky.” But, he added, “we don’t have all the details yet.”

Biden said the administration is concerned about reports that U.S. citizens were on the plane.

“We’re now working every minute to try to confirm those reports as I speak,” he said.

Philip Rucker contributed to this report.