Obama boosts Islamic State fight, asks Europe to do the same

President Barack Obama also cast his decision to send 250 more troops to Syria as a bid to keep up the pressure on ISIS

Evoking history and appealing for solidarity, President Barack Obama on Monday cast his decision to send 250 more troops to Syria as a bid to keep up "momentum" in the campaign to dislodge Islamic State (ISIS) extremists.

He pressed European allies to match the US with new contributions of their own.

Obama's announcement of the American troops, which capped a six-day tour to the Middle East and Europe, reflected a steady deepening of US military engagement, despite the president's professed reluctance to dive further into another Middle East conflict.

As Obama gave notice of the move, he said he wanted the US to share the increasing burden.

Obama discussed the ISIS fight with British Prime Minster David Cameron, German Chancellor Angela Merkel, French President Francois Hollande and Italian Prime Minster Matteo Renzi.

The president formally announced the new troop deployment in a speech about European unity and trans-Atlantic cooperation, a running theme of his trip.

Speaking in Germany, he evoked the continent's history of banding together to defeat prejudice and emerge from the "ruins of the Second World War".

"Make no mistake," Obama said. "These terrorists will learn the same lessons as others before them have, which is, your hatred is no match for our nations united in the defence of our way of life," he added.

The rhetoric belied an underlying frustration in his administration about allies' contributions to the US-led fight in Syria and neighbouring Iraq.

Although the coalition includes some 66 nations, the US has conducted the vast majority of the air strikes, and there has been little appetite by other nations to send in ground troops of their own.

The president recently rattled leaders in Europe and the Middle East by describing allies as "free riders". He made a passing reference to that complaint on Monday, as he noted that not all European allies contribute their expected share to NATO. He said, "I'll be honest, sometimes Europe has been complacent about its own defence."

On stops in Riyadh, London and Hannover this week, Obama repeatedly pushed allies for more firepower, training for local forces and economic aid to help reconstruct regions in Iraq that have been retaken from Islamic State control but are still vulnerable. Obama appeared to come up short in Riyadh, when he met with Arab allies.