MUSCAT, Oman (AP) -- U.S. Defense Secretary Jim Mattis on Sunday warned the Syrian government not to use chemical weapons in its civil war and said the Trump administration has made it clear that it would be "very unwise" to use gas in attacks.
Mattis told reporters traveling with him to the Middle East that he was disturbed by reports of civilian casualties from bombings by Syrian President Bashar Assad's forces.
"Right now we're getting reports — I don't have evidence that I can show you — but I'm aware of the reports of chlorine gas use," he said before arriving Sunday in Oman.
The U.S. responded militarily last year to reported Syrian government use of sarin gas, and Mattis was asked whether the administration is now considering retaliating for chlorine gas use.
"I'm not going to strictly define it. We have made it very clear that it would be very unwise to use gas" as a weapon, Mattis said.
He said the latest reports of Syrian government forces killing civilians in eastern Ghouta show that troops are "at best indiscriminately" attacking and "at worst targeting hospitals. I don't know which it is, whether they're incompetent or whether they're committing illegal acts or both."
The Pentagon chief said Russia, which intervened militarily in Syria to support the Assad government, could be complicit in the civilian casualties.
"Either Russia is incompetent or in cahoots with Assad," Mattis said. "There's an awful lot of reports about chlorine gas use or about symptoms that could be resulting from chlorine gas."
On Sunday, the Russian military said 52 civilians have evacuated from besieged eastern Ghouta suburbs of Syria's capital, Damascus. Russia and the Syrian government have accused rebels of blocking civilians from fleeing the violence.
The Britain-based Syrian Observatory for Human Rights monitoring group says an indiscriminate campaign of government and Russian air strikes and shelling has killed some 1,100 civilians in rebel-held eastern Ghouta over the past three weeks.
The U.N. estimates 400,000 civilians are trapped in the siege.
Syria's deputy foreign minister Faisal Mekdad denied opposition charges that government forces used poisonous gas in their attacks on some suburbs of Damascus.
Mekdad said at a news conference Saturday that insurgents groups in eastern Ghouta are preparing "to fabricate" more such attacks to blame the Syrian army.
While in Oman, Mattis planned to meet with the country's supreme ruler, Sultan Qaboos bin Said, on Monday. Oman is a longtime security partner of the U.S., though some question whether it is facilitating, or turning a blind eye to, the movement of Iranian weapons to Yemen to aid Houthi rebels.
Asked whether Oman is assisting Iran in this respect, Mattis said, "I'm not willing to say that." He said he expects to discuss Yemen with Qaboos. Oman has long-standing commercial and political ties to Iran, an Omani neighbor and U.S. nemesis.
Oman, which borders Yemen on the eastern edge of the Arabian Peninsula, has been ruled by the 77-year-old Qaboos since he took power from his father in a bloodless palace coup in 1970. After the 2011 Arab Spring, Qaboos is now the longest-serving Arab leader in the Middle East.
While ostensibly a member of the Saudi-dominated Gulf Cooperation Council, Oman has struck out on its own diplomatic path in the region. Oman was the site of secret talks between Iran and the U.S. that birthed Tehran's 2015 nuclear deal with world powers. Oman also has negotiated to help release Western detainees in Iran, as well as Yemen, in recent years.
Anthony Cordesman, a Middle East expert at the Center for Strategic and International Studies, said Saturday that Oman's role in the Persian Gulf is important to the U.S. at a time of sharp divisions among the Gulf Arab states, civil wars in Yemen and Syria, and growing Iranian influence in Iraq.
"The pattern in the Gulf is one of deep concern for the U.S.," Cordesman said.
Mattis said he also will visit Bahrain this week. It is home to the U.S. Navy's 5th Fleet.