US Secretary of State Rex Tillerson on Monday said that the US will stand up against anyone who commits crimes against humanity, referring to last week's chemical attack in Syria.
"We rededicate ourselves to holding to account any and all who commit crimes against the innocents anywhere in the world," Tillerson told reporters ahead of a G7 leaders meeting in Italy.
Tillerson is set to make his first diplomatic visit to Moscow later this week. He said the attack would not have been possible if the Russians did not fail to ensure the destruction of the Syrian regime's stockpile of chemical weapons.
However, the secretary of state said he accepted there was no evidence that Moscow itself had been involved in the attack in opposition-held Idlib that killed at least 80 people, including many children.
McMaster says he wants Assad out
US President Donald Trump's national security adviser has left open the possibility of more military action against Syria after the US on Friday fired 59 cruise missiles at the Shayrat air base in retaliation for the chemical attack.
However, General HR McMaster said the US was not seeking to act unilaterally to oust Assad. He said the US aims to defeat Daesh and remove Assad but only with the consent of Russia and its allies.
General McMaster also warned Russia to review its support for the Syrian regime.
"It's very difficult to understand how a political solution could result from the continuation of the Assad regime," General McMaster said.
"Now, we are not saying that we are the ones who are going to affect that change. What we are saying is, other countries have to ask themselves some hard questions. Russia should ask themselves, ‘Why are we supporting this murderous regime that is committing mass murder of its own population?'" he added.
McMaster made clear that the objective of last week's missile strike was to send a "strong political message to Assad."
Trump national security chief underlined that if Assad's regime continues to engage in atrocities against opposition forces with either chemical or conventional weapons, the US would respond with additional strikes. "We are prepared to do more," he said.
"The president will make whatever decision he thinks is in the best interest of the American people."