By Sarah N. Lynch

WASHINGTON (Reuters) – Trump administration officials on Sunday blamed Russian inaction for enabling a deadly poison gas attack against Syrian civilians last week as Secretary of State Rex Tillerson prepared to explain to Moscow a U.S. retaliatory missile strike.

Tillerson said Syria was able to execute the attack, which killed scores of people, because Moscow had failed to carry out a 2013 agreement to secure and destroy chemical weapons in Syria.

White House National Security Adviser H.R. McMaster said Syria’s “sponsors,” Russia and Iran, were enabling President Bashar al-Assad’s “campaign of mass murder against his own civilians.”

But Tillerson, who is expected to visit Moscow on Wednesday for talks with Russian officials, said on ABC’s ‘This Week’ program there was “no change” to the U.S. military posture toward Syria.

“I think the real failure here has been Russia’s failure to live up to its commitments under the chemical weapons agreements that were entered into in 2013,” Tillerson said.

“The failure related to the recent strike and the recent terrible chemical weapons attack in large measure is a failure on Russia’s part to achieve its commitment to the international community,” he added.

U.S. President Donald Trump ordered cruise missile strikes on a Syrian air base after he blamed Assad for the chemical attack, which killed at least 70 people, many of them children, in the Syrian town of Khan Sheikhoun. The Syrian government has denied it was behind the attack.

Speaking on “Fox News Sunday,” McMaster said the United States would take further action in Syria if necessary.

“We’re prepared to do more. In fact, we were prepared to do more two days ago,” McMaster said. “The president will make whatever decision he thinks is in the best interests of the American people.”

Russian President Vladimir Putin and Iran’s Hassan Rouhani said in a phone call that aggressive U.S. actions against Syria were not permissible and violated international law, the Kremlin said.

McMaster said Russian leaders were supporting “a murderous regime” and their actions would dictate the future of U.S.-Russian relations.

“Do they want it to be a relationship of competition and potential conflict,” McMaster said. “Or do they want it to be a relationship in which we can find areas of cooperation that are in our mutual interest?”

Tillerson stopped short of accusing Russia of direct involvement in planning or carrying out the attack, saying he had not seen “any hard evidence” to suggest Moscow was an accomplice to Assad.

But he said the United States expected Russia to take a tougher stance by rethinking its alliance with Assad because “every time one of these horrific attacks occurs, it draws Russia closer into some level of responsibility.”

(Reporting by Sarah N. Lynch and David Morgan; Editing by James Dalgleish)