U.S. Defense Secretary Jim Mattis is warning the Syrian government not to use chemical weapons in its civil war and says 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 Mideast 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 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.

Syria's deputy foreign minister Faisal Mekdad, denied opposition charges that government forces used poison gas in their attacks on some suburbs of Damascus, the capital.

Mekdad said at a news conference Saturday that insurgents groups in the 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., although 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 longstanding commercial and political ties to Iran, an Omani neighbor but a U.S. nemesis.

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.