Combined military strikes against Syria by America, British and French forces involved more targets and twice as many weapons as a similar attack launched nearly exactly a year ago, Secretary of Defense James Mattis said late Friday.
Mattis is asking that "responsible nations" join in condemning the Assad regime.
"We will not know, if this investigating team that goes in - if we get them in, if the regime will let them in - we will not know who did it", he said.
The presence of Russian troops and air defenses in Syria were among numerous complications weighing on Trump, who must also consider the dangers to roughly 2,000 American troops in the country if Russia were to retaliate for US strikes.
The United States, along with assurance from France and the United Kingdom, launched a response Friday against the regime of Syrian leader Bashar al-Assad days after his government allegedly used chemical weapons on its citizens. White House press secretary Sarah Sanders also said on Wednesday the intelligence community was still assessing the evidence but said, "the President holds Syria and Russian Federation responsible for this chemical weapons attack".
Mattis said that any further strikes would "depend on Mr. Assad, should he decide to use more chemical weapons in the future".
Adam Smith, the ranking Democrat on the House Armed Services Committee, said he sees no legal justification for a USA strike in Syria, absent explicit authorisation by Congress. "At the same time, we keep our foot on the neck of ISIS until we suffocate it", he said referring to the Islamic State of Iraq and the Levant (ISIL, also known as ISIS) group.
Russian Federation on Thursday warned the U.S.to avoid any steps that could destabilize the situation there.
The ambassador said that Russian specialists went to Douma to inspect the situation, but could not find any evidence of a chemical attack: "Locals were interviewed about the cessation of resistance to the fighters".
Mattis listed two concerns about directly attacking Assad regime targets: "There's a tactical concern of innocent civilians - that we don't add to any civilian deaths ..."
While last year's strikes were done unilaterally, and hit one target - a Syrian regime airfield that housed about 17 percent of its air force - Dunford said Friday's strikes were conducted with two allies on multiple sites and would "result in a long-term degradation of Syria's capability to develop chemical weapons".
"The targets that were struck and destroyed were specifically associated with the Syrian regime's chemical weapons program".
The White House said on Thursday (Friday NZT), that Trump would consult further with allies. "But at the same time, it was a heavy strike".
The U.S. military official claims it will become "more difficult" to obtain evidence to back up the claims that are being used to justify escalations towards war with Syria.
At the House hearing, Democrat representative Tulsi Gabbard disputed Trump's legal authority to act without congressional authority and suggested a U.S. strike would lead to war with Russian Federation.