US President Barack Obama on Tuesday flew directly into the worst East-West storm since the Cold War, heading first for Estonia, where he will warn Russia to keep its hands off former Soviet states now in NATO.

With the crisis over Ukraine fast developing into a dangerous confrontation between the Western alliance and the Kremlin, Obama boarded Air Force One for the flight across the Atlantic on a trip which will also include a critical NATO summit in Wales.

Another foreign policy saga stalking Obama -- the rise of the Islamic State (IS) in Syria and Iraq -- also took another turn for the worse Tuesday, with the apparent murder of US journalist Steven Sotloff depicted in a new video from the radical Sunni group.

The White House says Obama's mission in Estonia, where he will also meet the leaders of Latvia and Lithuania, is simple -- to put Russia on notice that it must not attempt the kind of intimidation and infiltration of NATO's former Soviet members that it has carried out in Ukraine, a non-NATO member.

"NATO has played a leading role and produced ample evidence to indicate that Russia has intervened in ways that grossly violate the territorial integrity of the independent nation of Ukraine," said White House spokesman Josh Earnest.

"It's not OK for large countries to flagrantly violate the territorial integrity of their smaller neighbors."

- Symbolic speech -

Obama headed first to the Estonian capital Tallinn, where he will give a highly symbolic speech on Wednesday and meet Estonian President Toomas Hendrik Ilves, Lithuanian President Dalia Grybauskaite and Latvia's president Andris Berzins.

The three Baltic states, which joined NATO in 2004, spent five decades under Soviet rule, and feel themselves vulnerable to the kind of covert warfare pioneered by Moscow in Ukraine owing to their substantial Russian minorities.

Senior Obama aides say the president will make clear that the United States regards its Article Five commitments on the collective self-defense of NATO members as sacrosanct, and will not tolerate any Russian moves on American allies.

To steel Obama's message ahead of its summit at the Celtic Manor resort in Wales, NATO has unveiled plans for a rapid reaction force of thousands of troops to reinforce the alliance's eastern flank.

Russia reacted angrily to the move, saying it was evidence of NATO's desire to aggravate tensions with Moscow, and vowed to reinforce its own defenses as a result.

Ukraine's forces suffered new reverses on Tuesday against Moscow-backed rebels after accusing Russia of sending 1,000 regular troops into its eastern regions.

Russia denies the allegations and says it is trying to drive a land corridor towards the Crimea -- a Ukrainian region which it annexed earlier this year.

- 'Critical moment' -

Analysts say the trip to Europe marks a crucial moment in Obama's presidency -- at a time when his foreign policy is under withering attack from rivals for failing to quell the territorial ambitions of both Russian President Vladimir Putin and IS radicals.

"This is a very dramatic backdrop, where Europe's security environment has turned upside down," said Heather Conley, of the Center for Strategic and International Studies in Washington.

"NATO had never thought nine months ago that it would be focusing on a robust collective defensive posture in northern Europe, but that's exactly where we are today."

Obama left Washington without commenting on a video purported to show the IS execution of US journalist Sotloff, following the killing last month of another American reporter, James Foley.

The horrifying footage will increase pressure on Obama to come up with a swift and robust military plan to strike at IS strongholds inside Syria, after he caused a political storm last week by admitting he did not yet have a strategy to do so.

What to do about IS, and US efforts to build an international coalition to take on the group, will be a key feature of the NATO summit on Thursday and Friday.

The meeting will also consider the impact of Afghanistan's electoral crisis on plans to leave a small residual force in the country after alliance combat troops leave by the end of the year.