ANALYSIS: Trump administration unable or unwilling to stop new Syria bloodshed

The U.N. Security Council called Saturday for a ceasefire in Syria.

"The president put the Assad regime on notice some time ago, and we're continuing to echo that message," White House press secretary Sarah Sanders said Monday. "I'm not going to broadcast what we may or may not do, but I think they should absolutely take it very seriously."

But unless he wades more heavily into the fight, or does the heavy lift of pursuing some creative diplomatic solutions, the U.S. will remain on the sidelines, unable to halt the horrific scenes from unfolding, experts warn.

Russia's power there is also evident at the United Nations. While the Security Council unanimously approved the demand for a cessation of hostilities Saturday, the final resolution was filled with holes.

The Trump administration viewed it as a small victory, given the unanimous vote, and in hopes it presented a united front against Assad and Russia in particular, according to officials.

In the face of the continued violence, the U.S. and its allies appeared to have no recourse. Secretary of State Rex Tillerson has admitted as much before about Syria, noting there was little the U.S. could do to stop Russia and Assad.

"I don't think we can take action to make them do more. They've got to decide that it's in their interest to do it," Tillerson told ABC News on February 7, as the bombing in east Ghouta first escalated and after a series of reported chemical weapons attacks.

There are steps the U.S. could take to convince Russia of that, officials have said. But so far, the State Department won't say whether or not Tillerson has called his Russian counterpart, Foreign Minister Sergei Lavrov, saying such calls are private diplomatic conversations.

But that begs the question of what the administration is doing, beyond the vote at the U.N.

"I don't know what some of you expect us to do," State Department spokesperson Heather Nauert said Thursday after repeated questions during the department's briefing. "We have a full range of options through the interagency that are available to us if we want to or if we should need to use that. Our best tool, what we do out of this building is an attempt at diplomacy, an attempt to shine a spotlight on things that are taking place around the world."

There has been extensive news coverage of the situation in Ghouta, but little has changed. Nauert wouldn't say what those U.S. tools on the table are - but when the Assad regime used sarin gas last April, the Trump administration conducted airstrikes, hitting the Syrian airbase where Assad's jets armed with chemical weapons had taken off.

Many of Trump's allies and opponents cheered what they called the president's strong action. But ten months later, Assad has used chemical weapons again - and still employs other brutal tactics like barrel bombs, improvised bombs filled with explosives and shrapnel dropped from helicopter or airplane.

There is little appetite among the American public for increased military involvement in the conflict. But others have called for the U.S. to take other steps, such as sanctions on Russia or withholding funds for U.N. aid.

"Let’s stop funding UN agencies charged with delivering humanitarian aid inside Syria if the Syrian government continues to block the aid," offered Amb. Robert Ford, the last U.S. ambassador to Syria, and Mark Ward, the former director of the Syria Transition Assistance and Response Team that coordinated U.S. aid to Syria from Turkey, in a recent op-ed. "The UN might complain, but our taking such a bold step would strengthen their hand with the government in Damascus and its allies."

In the interview, Ford, a fellow at the Middle East Institute and Yale University, added that the U.S. could also target Iran's resupply missions that bolster the regime by ensuring they can't fly over U.S.-controlled eastern Syria or neighboring countries allied with the U.S.

But even such creative diplomatic maneuvers would not change the battlefield reality.

"It's a war, and if you want to promptly change things, it's the military," not U.N. resolutions that Russia, Iran, and Assad consistently violate, Ford said. "This is going to be very painful to watch... [but] short of stern U.S. military intervention, there is not a whole lot they can do."

To that point, Nauert would not even say whether the administration believes the U.S. has a responsibility to ease the violence and protect civilians.

"Some of these things are things that we will not be able to talk about," she said. "Some of these things will become intelligence matters and other issues as well."

Instead, she noted statements from the White House, State Department, and U.S. Ambassador Nikki Haley's office at the U.N., condemning Assad's assaults and reported chemical weapons use and calling out Russia for its "unique responsibility" to rein in the regime.

Russia has not done so.

In the face of the U.N. resolution, Russia instead is implementing a "humanitarian pause" from 9:00 a.m. to 2:00 p.m. each day "at the order of the Russian president [Vladimir Putin] in order to avoid civilian casualties," Russian Defense Minister Sergei Shoigu said Monday - a flex of Russian influence that the U.S. does not have.

All of this is not a new problem. For six years, President Barack Obama struggled with how involved to get in Syria, wavering between fear of being pulled into another Middle East conflict and an urge to protect innocent lives.

Most famously, he backed down from his "red line" to respond if Assad used chemical weapons, after the Syrian leader did so in Ghouta - the same suburb now being bombed into submission. But he also faced a similar dilemma when Assad and Russian jets pummeled Syria's once-largest city, Aleppo.

Despite international outcry and a flurry of diplomatic attempts then, the Obama administration was ultimately powerless to stop Assad's march to victory in Aleppo. Instead, the U.N. Security Council passed a similar resolution calling for humanitarian access - and the Assad regime and Russia bombed aid convoys, without consequence.

But to Obama, not getting involved was a sign of strength, arguing that Putin would get "bogged down" in Syria.

"Rather than being able to count on their support and maintain the base they had in Syria, which they've had for a long time. Mr. Putin now is devoting his own troops, his own military just to barely hold together by a thread his sole ally," he told CBS's "60 Minutes" in October 2015.

Less than two-and-a-half years later, Assad is in control of 50 percent of the country again, by State Department estimates, and is assaulting the remaining rebel enclaves in east Ghouta and northwestern Idlib province.

ABC News's Rym Momtaz and Tanya Stukalova contributed to this report.