US President Barack Obama will use a NATO summit next year to write the "final chapter" of America's longest war, in Afghanistan, hoping to cement a legacy rooted in ending foreign conflicts.

Obama announced that alliance leaders would meet next year at a time and place to be named, to consider the limited training and assistance mission due to roll out once NATO combat troops leave at the end of 2014.

But, after meeting NATO Secretary General Anders Fogh Rasmussen for talks on ending the Afghan war, Obama dodged questions about Syria, as he seeks to keep his nation out of another costly venture in the Muslim world.

Obama has used previous NATO summits as action-forcing mechanisms to coalesce member states around US policy: the 2014 withdrawal strategy was enshrined in the run-up to the 2012 meeting in his hometown of Chicago.

This time, Obama will seek commitments and input to a post-2014 bid to keep Afghan security forces effective and in control of their borders to ensure Afghanistan will not revert to a haven for terrorism.

The summit will "underscore this final chapter in our Afghan operations," Obama said.

Rasmussen argued that "our goal is in sight" in Afghanistan, despite uncertainty over the country's fate when foreign troops leave, security worries and the resilient Taliban.

He defined the goal as "an Afghanistan that can stand on its own feet -- but the Afghans will not stand alone."

Obama built a political brand on opposing a war -- in Iraq -- that became unpopular and after ending the Afghan war, hopes history will see him as the president who extricated America from costly foreign misadventures.

A week ago, Obama laid out a plan to wean America from its "perpetual" war on terror that has raged since 2001, saying the country was at a crossroads, though pledged to pursue metastasizing Al-Qaeda-inspired terror franchises.

This context partly explains Obama's reticence to thrust Washington more overtly into the civil war in Syria, either by arming rebel groups opposing President Bashar al-Assad's regime or by mounting a no-fly zone.

Faced with no good options that would be sure to topple Assad, preserve the integrity of the Syrian state, usher in a political transition or avoid the risk of US "mission creep," Obama has stayed mostly on the sidelines.

He has provided humanitarian aid and non-lethal equipment to refugees and some selected rebel groups, but Washington fears any weapons it may send could end up in the hands of violent extremists in the fractured opposition.

Obama sidestepped questions shouted by reporters about Syria after meeting Rasmussen, and is pinning hopes on a diplomatic track emerging out of a US-Russia-led peace conference, which is yet to be scheduled.

But while he faces frequent demands for action from the Washington media and high profile political foes like Senator John McCain, who visited Syrian rebels this week, Obama's strategy appears solidly underpinned by public opinion.

Polling consistently shows that Americans are weary of overseas wars -- a mood that Obama harnessed when he won the presidency in 2008 -- and see no clear advantage or logic in intervening in Syria.

A Gallup poll showed that only 24 percent say Washington should take military action to end the war in Syria while 68 percent say it should not.

And despite their reticence to get involved, only 27 percent of those polled on May 28-29 thought economic and diplomatic means could end the conflict.

A Washington Post/ABC News poll in December had earlier found that 73 percent believed the US military should not get involved in Syria.

But with limited military options, and with the diplomatic track ahead of the possible peace conference in Geneva tortured, Washington is reduced to a largely rhetorical response as the conflict deepens.

Secretary of State John Kerry, who was in the Rasmussen talks, said that Russia's planned delivery of S-300 air defense missiles to its ally Assad was "not helpful."

Obama spokesman Josh Earnest, meanwhile, fumed about Assad from the podium in the White House press room.

"This is the guy who has demonstrated his willingness to slaughter, without any conscience, innocent people by the thousand," he said.