President Obama firmly denounced the downing of a Malaysian jetliner over Ukraine, calling the incident an "outrage of unspeakable proportion" and saying for the first time that U.S. authorities had determined that a rocket attack was responsible for the crash that killed 298 people.

The president stopped short of blaming pro-Russian separatists for the deadly airliner crash but said U.S. authorities had determined that the shot came from an area in Ukraine controlled by Russian-backed separatists and are working round the clock to determine exactly who is responsible. He also vowed that there would be consequences for the assault, mistaken or not.

"Evidence indicates that the plane was shot down by a surface-to-air missile that was launched from an area that is controlled by Russian-backed separatists inside of Ukraine," he told reporters in a statement in the White House's Brady press briefing room.

"We also know that this is not the first time a plane has been shot down in eastern Ukraine. Over the last several weeks Russian- backed separatists have shot down a Ukrainian transport plane and a Ukrainian helicopter, and they claimed responsibility for shooting down a Ukrainian fighter jet."

"Moreover, we know that these separatists have received a steady flow of support from Russia," he said. "This includes arms and training. It includes heavy weapons. And it includes anti-aircraft weapons"

Obama had tough words for Russian President Vladimir Putin, saying that despite repeated entreaties from the U.S. and others for him to tell the separatists to stand down, he has continued to supply them with weapons and support.

"If Mr. Putin makes a decision that we are not going to allow heavy armaments and the flow of fighters into Ukraine and across the Ukrainian-Russian border, than it will stop. If it stops, then the separatists still have the capacity to enter into negotiations that Mr. Putin himself says he wants," Obama said.

"He has the most control over that situation and so far, he is not exercised it," Obama said.

At the beginning of his remarks, Obama confirmed the death of one American in the crash, Quinn Lucas Schansman, and said the nation's thoughts and prayers are with his family and all families whose loved ones died.

"Nearly 300 innocent lives were taken — men, women, children, infants — who had nothing to do with the crisis in Ukraine," he said. "Their deaths are an outrage of unspeakable proportion."

Twenty-three American passengers are feared dead in the crash, although U.S. authorities had not confirmed those numbers as of press time. In addition, officials said most of the passengers were from the Netherlands, but citizens on board were from more than a dozen countries, including Australia, Malaysia, the United Kingdom, Germany, Belgium, the Philippines and Canada.

"I want the Dutch people to know that we stand with you shoulder-to-shoulder in our grief and in our absolute determination to get to the bottom of what happened," Obama said.

Obama's remarks Friday morning followed 24 hours of shock and finger-pointing over the downing of the civilian passenger jet. Ukrainian officials blamed pro-Russian separatists fighting in eastern Ukraine, while Russian President Vladimir Putin on Thursday said Ukraine bore responsibility for the tragic crash.

Obama's forceful statement Friday morning came after an onslaught of criticism for his response the day before. Republicans blasted the president for sticking with his predetermined Thursday schedule, which included two fundraisers in New York City on in the evening, and earlier in the day continuing with a joke-filled infrastructure speech in Delaware after briefly calling the crash a likely “tragedy.”

“With all that is going on, the last thing the President should be doing is fundraising right now. Now is a time for leadership,” Republican National Committee Chairman Reince Priebus tweeted.

Sen. John McCain, R-Ariz., was far harsher, during an interview Thursday night on Fox News, saying the president was "AWOL" in time of crisis.

Late Thursday night the White House called for an immediate cease-fire in eastern Ukraine to allow international investigators access to the crash site.

In his statement Friday morning, Obama reiterated the need for a "credible international investigation."

"The U.N. Security Council has endorsed this investigation, and we will hold all its members, including Russia, to their word," he said.

Pressure on Russia began to mount Friday as the United Nations Security Council met to discuss the incident. Before Obama's remarks, U.S. Ambassador to the United Nations Samantha Power delivered an emotional address to her counterparts and called for a "full, credible, and unimpeded international investigation" into the crash to begin immediately.

Putin on Friday called the crash an “awful tragedy,” said he was in contact with Ukrainian President Petro Poroshenko and joined calls for an immediate ceasefire.

“Direct talks between the opposing sides must be established as soon as possible. All sides in the conflict must swiftly halt fighting and begin peace negotiations,” he said at a meeting with Russian Orthodox Church leaders.

"It is with great concern and sadness that we are watching what is happening in eastern Ukraine. It's awful, it's a tragedy."