Americans roundly reject U.S. military action to end Syria's civil war, with opposition to an intervention unshaken by recent allegations that Syrian government forces have used chemical weapons.

Two in three Americans (68 percent) say the U.S. military should not attempt to end conflict in Syria if diplomatic efforts fail, according to a Gallup poll released Friday, with opposition cresting over 60 percent among Republicans, Democrats and independents alike. The verdict is similar to a December Washington Post-ABC News poll, which found 73 percent saying the military should not get involved in the situation in Syria. Despite general opposition, the Post-ABC poll found wide support for action if the country used or lost control of its chemical weapons stockpile.

The Gallup survey is the clearest measure of support for military action, if diplomacy fails, since Britain and France accused Syria of using chemical weapons in April, with the United States providing additional evidence to the United Nations in May. Immediately after allegations were raised, a New York Times/CBS News poll found 62 percent of Americans saying the United States has no responsibility to do something about the violence. While President Obama has described use of chemical weapons as a "red line," his administration has been less resolute than European allies, saying that while Syria probably used weapons, it could not confirm details.

Americans also express broad uncertainty. Only 16 percent were “certain” Syria had used chemical weapons to kill civilians in a CNN/ORC poll released this week; far more, 67 percent, said it was “likely, but not certain. In addition to reflecting the U.S. response, the weak conviction may reflect a lack of attention: Only 43 percent of Americans followed news about chemical weapons accusations in late April "very" or "fairly" closely according to a Pew Research Center poll, and earlier this month just 41 percent were paying attention to broader violence in Syria.

With public attention low, it's unclear whether mounting allegations that Syria used chemical weapons will translate into support for action. The late-April Pew Research survey found more Americans favor than oppose military action if Syria's use of chemical weapons is confirmed, and the Post-ABC poll last year found majority support for several specific actions if the nation uses weapons or loses control of its cache.

The Gallup poll was conducted May 28 to 29 among a random national sample of 1,011 adults reached on conventional and cellular phones. The margin of sampling error for the overall results is plus or minus four percentage points. Full details here.

Scott Clement is a pollster with Capital Insight, the independent polling group of Washington Post Media. Capital Insight pollster Peyton M. Craighill contributed to this post.