WASHINGTON (CNN) -

No matter who did it, shooting down a commercial airliner with almost 300 people aboard gravely escalated an already dangerous conflict in eastern Ukraine.

The airline disaster Thursday turned what had been regional unrest into a full-blown international conflict, with questions of responsibility, motive and reparations for the families of victims who might come from all over the world.

More than that, the tragedy appeared certain to worsen a so-far diplomatic showdown regarding Ukraine with Russia on one side and the United States and the European Union on the other.

Already, hawkish Sen. John McCain of Arizona called for a bigger U.S. role if Russia was involved.

"This is a major catastrophe that could potentially have very, very far-reaching implications and geo-strategic meaning right now," said CNN Chief International Correspondent Christiane Amanpour.

What happened?

U.S. intelligence concluded the Malaysia Airlines Boeing 777 flying from Amsterdam to Kuala Lumpur was shot down over Ukraine, a senior U.S. official told CNN, adding it wasn't yet clear who did it.

Finger-pointing began almost immediately.

Ukraine blamed Russian-backed separatists fighting the Ukrainian military in the Donetsk region where the plane went down. The separatists said they lacked the firepower to hit a plane flying at more than 30,000 feet and accused Ukraine of doing it.

Military experts noted Russia's military, with forces again massing at the Ukraine border, has the missile systems capable of such a strike.

They pointed out that separatist forces recently shot down a Ukrainian transport plane at 21,000 feet, which was out of range of known separatist weaponry.

After a classified briefing later Thursday, McCain told reporters that "the separatists could have only gotten that capability from Russia."

Huge repercussions

Does it matter who did it? Yes and no.

The repercussions are huge in any case, but will differ depending on what happened.

Regardless of who fired the missile, the impact of such an air disaster means investigations, reports, lawsuits and other bureaucratic responses that continue for years.

The international component of passengers potentially hailing from multiple countries adds further complexity, along with the location in a region of low-level warfare in recent months.

Already, some international airlines announced they would avoid Ukrainian air space for now, and U.N. Secretary General Ban Ki-moon called for a "full and transparent international investigation."

"If it transpires that the plane was indeed shot down, we insist that the perpetrators must swiftly be brought to justice" Malaysian Prime Minister Najib Razak said.

U.S. response

President Barack Obama and Vice President Joe Biden spoke by phone with Ukrainian President Petro Poroshenko to offer assistance, and Biden said a team of U.S. aviation experts would head there right away.

In language that seemed intended to stoke an emotional response, Biden said that while "we don't have all the details," the plane "apparently had been shot down. Shot down. Not an accident. Blown out of the sky."

If separatists using Russian firepower or Russian forces did it, the United States and European Union would come under pressure to take tougher steps against Moscow.

So far, they have imposed sanctions on Russia and provided economic help and other aid to Ukraine. On Wednesday, Obama announced further sanctions due to Russia's failure to take steps to end the Ukraine conflict.

"This would have the most profound repercussions," McCain told CNN of possible Russian responsibility. Later, he told reporters the United States should "arm the Ukrainian military, which we have shamefully not done, increase sanctions to a dramatic degree" and treat President Vladimir Putin and Russia "as a pariah nation, which if, they are responsible for this, they deserve."