With global outrage mounting over a chemical attack on a rebel-held town in Syria, the Pentagon on Monday weighed its options for a retaliatory strike. Military action against the regime of Bashar Assad seemed likely, after President Donald Trump warned of a "big price to pay" and spoke of imminent "major decisions" within the next 48 hours, said an Agence France Presse report on Monday. Defense Secretary Jim Mattis said he wont rule anything out militarily. But thanks to the Trump administrations whipsawing messaging over whether America will even stay in Syria, and the dangerous complexities of the multi-national conflict, the Pentagons options appeared limited. The attack on the rebel-held Syrian town of Douma killed at least 48 people Saturday after a "poisonous chlorine gas attack" in Eastern Ghouta, rescuers and medics said. By Monday, the United States and France had promised a "strong, joint response" and Britain, too, joined a growing chorus demanding action. Syria and its ally Russia have dismissed allegations that the attack was carried out by regime forces as "fabrications”. Perhaps the biggest risk for Pentagon planners is Russia, and its large presence which since late 2015 has been deeply enmeshed with Assads forces. "The US has to be very careful not to accidentally strike Russian targets or kill Russian advisors," Ben Connable, a senior political scientist at the RAND Corporation, told AFP. "That significantly limits the number of options available to the United States, because the Russians are embedded in many cases with the Syrians." Trump made a rare personal criticism of Russian President Vladimir Putin following Saturdays attack, a break from his reluctance to single out the strongman by name as he has sought better coordination with Moscow in the Syria crisis. "President Putin, Russia and Iran are responsible for backing Animal Assad," Trump wrote in a tweet, according to AFP. After a deadly sarin gas attack on the rebel-held town of Khan Sheikhun that killed scores of people in April last year, Trump quickly ordered a retaliatory strike. The US military blasted 59 Tomahawk missiles at Syrias Shayrat air base, which the Pentagon said Assads regime had used to launch the deadly chemical attack. The action won Trump bipartisan praise because it was seen as limited in scope and designed to respond to a specific incident, rather than pulling America deeper into Syrias war. "The president responded decisively when Assad used chemical weapons last year," Republican Senator John McCain, a frequent Trump critic said. "He should do so again, and demonstrate that Assad will pay a price for his war crimes." Jennifer Cafarella, a Syria analyst for the Washington-based Institute for the Study of War, said Trumps administration needs to figure out what its long-term goal is in Syria as it weighs its military options. She described a few potential military responses, including a tactical strike such as the one last year, or a broader attack on regime air forces including taking out radar and air-defense systems, and hitting multiple air bases. "I expect the question from the Pentagon to the civilian leadership is what is the goal," Cafarella told AFP. She said another option likely under consideration is to tackle Iranian-backed militias in Syria. Such a move would not be in direct response to the latest chemical attack, but would signal a willingness to curtail Iranian influence in Syria. "We want to ask whether the president is going to broaden his response in order to also punish Assads backers Russia and Iran," she said. Another possibility is Trump asking an ally to conduct military action against the regime. On Monday, French President Emmanuel Macron said he and Trump had shared information "confirming" toxic weapons were used in Douma, without elaborating. Trump on Monday began working with his new national security advisor, John Bolton, a staunch hawk on Iran and American military intervention in general, so the presidents outlook on Syria -- and whether he still wants to withdraw the roughly 2,000 US forces -- may soon morph again.