WASHINGTON (Reuters) - U.S. Defense Secretary Jim Mattis told Congress on Thursday he believes there was a chemical attack in Syria, adding that the United States wants inspectors on the ground soon since the job of collecting evidence becomes more difficult as time passed.

Mattis, addressing a hearing of the House of Representatives Armed Services Committee, declined to discuss U.S. military planning on Syria.

But he acknowledged two main concerns as Washington mulls potential action against Syrian President Bashar al-Assad’s forces: protecting civilians and avoiding triggering a military escalation that gets “out of control.”

Fears of confrontation between Russia and the West have been running high since U.S. President Donald Trump said on Wednesday that missiles “will be coming” after the suspected chemical weapons assault in the town of Douma on April 7, and lambasted Moscow for standing by Assad.

Mattis, in some of his strongest comments on Syria to date, said he had been convinced of a chemical attack in Syria.

“I believe there was a chemical attack and we are looking for the actual evidence,” Mattis told lawmakers, adding he wanted inspectors in Syria “probably within the week.”

“As each day goes by -- as you know, it is a non-persistent gas -- so it becomes more and more difficult to confirm it.”

At the United Nations, Syrian U.N. Ambassador Bashar Ja’afari told reporters that two teams of investigators from the global chemical weapons watchdog are due to arrive in Syria on Thursday and Friday to look into the Douma incident.

Mattis also accused Russia of being complicit in Syria’s retention of chemical weapons, despite a 2013 deal requiring Syria to abandon them that Moscow helped broker.

He noted that Trump’s predecessor, Barack Obama, sought to address Syria’s use of chemical weapons by striking that deal -- which averted U.S. military action against Syria.

In doing so, Obama was “enlisting the Russians, who now, it shows, were complicit in Syria retaining those weapons – Assad retaining them.”

“And the only reason Assad is still in power is because of the Russians’ regrettable vetoes in the U.N., and the Russian and the Iranian military,” he said.