A Malaysian flight carrying 298 people crashed near the Russia-Ukraine border on Thursday, raising immediate worries it had been shot down by Russian militants.
If Russian rebels are behind the downed plane, it will raise diplomatic pressure on Putin to cut any ties with the militants. The U.S. has blamed Russia's government for the fighting in Ukraine, and the Obama administration on Wednesday announced a new round of sanctions on Russia.
President Obama, who was out of Washington for much of the day to deliver a speech on transportation in Delaware and to attend fundraisers in New York, spoke with Poroshenko on Thursday and offered U.S. assistance to international investigators.
Both Obama and Poroshenko emphasized during the call that "all evidence from the crash site must remain in place on the territory of Ukraine until international investigators are able to examine all aspects of the tragedy," according to the White House.
Obama also talked to Putin on Thursday, and the two discussed the downed plane during a phone conversation that also touched on the latest U.S. sanctions.
Later, Obama said the U.S. would "offer any assistance we can, to help determine what happened and why" to Ukraine's government.
"I’ve directed my national security team to stay in close contact with the Ukrainian government," Obama said.
Following his discussion with Kerry, Obama phoned members of his national security team, including CIA Director John Brennan, White House chief of staff Denis McDonough, and Deputy Director of National Intelligence Stephanie O'Sullivan.
"The president was briefed on our ongoing efforts to support the Ukrainian government and a prompt international investigation into what took place," Earnest said. "The president directed his national security team to continue offering whatever assistance is necessary to advance the international effort to determine what happened."
While Biden and other administration officials were careful not to directly implicate Russian separatists, Hillary Clinton, Obama's former secretary of State and a likely White House contender in 2016, suggested the plane had been downed by Russian militants using Russian equipment.
"There does seem to be some growing awareness that it probably had to be Russian insurgents," Clinton said on "The Charlie Rose Show" Thursday. "The equipment had to have come from Russia. What more the Russians may or may not have done we don't know."
Biden told reporters in Detroit he made an offer of assistance to Poroshenko in a phone call following the "tragic event that's taken place on the Ukrainian border," which the Ukrainian leader accepted.
An administration official said that technical assistance, including help determining the cause of the crash and analysis of the plane's black box recorder "would initially be on the table following an incident like this."
It is unclear if there were any Americans on board the flight. As of Friday morning, the nationalities of only four people had not been identified, and no Americans had been confirmed dead.
Obama also called Prime Minister Mark Rutte of the Netherlands on Thursday evening to express condolences for the death of 189 Dutch citizens aboard Malaysia Airlines Flight 17.
— This story was updated at 9:42 a.m. Friday.
— Alexander Bolton and Jesse Byrnes contributed to this story.