WASHINGTON (WASHINGTON POST) - The Trump administration is weighing military retaliation against the Syrian government following a chemical attack on Tuesday (April 4) that killed scores of civilians, but it is complicated by the presence of Russian forces in the country and concerns about US troops deployed in Syria in the campaign against the Islamic State in Iraq and Syria (ISIS), according to US officials.

The Pentagon is in the process of presenting options to the White House on potential military responses, which could include strikes on Syrian military targets and actions designed to ground the Syrian air force.

Senior White House officials met on the topic on Wednesday evening in a session that lasted into early Thursday, and Defence Secretary Jim Mattis, Secretary of State Rex Tillerson and Army Lt. Gen. H.R. McMaster, the national security adviser, have communicated repeatedly about the issue since Tuesday's attack, the officials said.

The Joint Chiefs of Staff were expected to meet early Thursday evening to discuss potential strike options, one military official said. One of the most likely is Tomahawk missiles launched from Navy ships in the Mediterranean Sea.

Some officials urged immediate action, warning against what one described as "paralysis through analysis." But others were concerned about second- and third-order effects, including the response of Russia, which also has installed sophisticated air-defence systems in Syria, according to the officials, who spoke on condition of anonymity to discuss internal deliberations.

Mattis called the attack a "heinous act" that would be "treated as such," and President Donald Trump suggested that he was deeply affected by the sight of the children and "beautiful babies" who were killed. "That crosses many, many lines," Trump said. "Beyond a red line - many, many lines."

Tillerson on Thursday did not rule out military options to respond to an attack that he said violates international agreements and norms. He also suggested that the United States and other nations would consider somehow removing Syrian President Bashar al-Assad from power, but he did not say how.

"We are considering an appropriate response for this chemical weapons attack," Tillerson said in Palm Beach, Florida, where Trump will meet with Chinese President Xi Jinping."It is a serious matter. It requires a serious response," Tillerson said.

Asked about the future of the Syrian strongman, who has withstood a multi-faceted war now in its seventh year, Tillerson appeared to change tack from earlier Trump administration statements about Assad.

"Assad's role in the future is uncertain, clearly," Tillerson told reporters. "With the action he's taken, it would seem there would be no role for him to govern the Syrian people."

Asked whether the Trump administration would lead a coalition to remove Assad, Tillerson replied: "Those steps are underway."

It was not immediately clear what he meant, either about a coalition or about removing a foreign leader. He suggested, however, that he was speaking about a long process and not some sudden military move.

"The process by which Assad would leave is something that I think requires an international community effort to both first defeat ISIS within Syria to stabilise the Syrian country, to avoid further civil war, then work collectively with our partners around the world on political protests that would lead to Assad leaving," said Tillerson.

He had said last week that Assad's political future was a matter for the Syrian people to decide, and UN Ambassador Nikki Haley said that defeating the ISIS was the higher priority.

The Obama administration had insisted that Assad could never remain in any post-war Syria, and it supported rebel groups that have tried unsuccessfully to oust him.

Also on Thursday, a senior State Department official said Tillerson spoke on the phone with Russian Foreign Minister Sergei Lavrov about the chemical attack on civilians in Syria. "We sought the Russian analysis or readout of what they thought had happened," the official said.

The United States has a broad arsenal already in the region, should Trump decide to attack, including dozens of strike aircraft on the USS George H.W. Bush, an aircraft carrier that is deployed to the Middle East and accompanied by guided-missile destroyers and cruisers that can also launch Tomahawk cruise missiles.

Additionally, an amphibious naval force in the region that includes the 24th Marine Expeditionary Unit could muster Harrier jets and Cobra gunships. The Pentagon also has scores of aircraft in the region flying operations every day against the ISIS, including from Incirlik air base to the north in Turkey.

The US Central Command has had plans for striking the Syrian government for years, but those plans would potentially need to be tailored to the precise goals of a strike in response to the chemical attack.

Officials said one consideration in examining a possible response is the presence of hundreds of American troops within Syria. Officials said there were measures in place to protect those troops but declined to give details.

US aircraft also would have to contend with a modest web of Syrian air defences and potentially more advanced types of surface-to-air missiles provided by Russia. One of Assad's more prevalent systems, the S-200, was used to target Israeli jets last month, but missiles were intercepted by Israeli defence systems. The S-200 has a range of roughly 186 miles (300km), according to US military documents, and can hit targets flying at altitudes of around 130,000 ft.

Russian S-300 and S-400 missiles, located primarily around Khmeimim air base in western Syria, have a shorter range than the S-200, but have more advanced radar systems and fly considerably faster than their older counterparts used by Syrian forces. The S-300 has a range of roughly 90 miles (145km) and could also be used to target incoming US cruise missiles.

Senator John McCain, R.-Ariz., chairman of the Senate Armed Services Committee, said in an interview on Thursday that he and Trump have discussed Syria in the past few days, but he said the president did not talk in particular about military options.

McCain, who has long backed a more aggressive response to the Assad government, laid out a short list of goals if the United States strikes. "Take out his air assets. No airplane should fly. No more barrel bombs. No more sarin gas," the senator said.

McCain said he was glad to see Trump's position on striking Assad changing."I know that he was deeply moved, as we all were, at the spectacle of this slaughter," McCain said.