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Russia's top diplomat accused the United States on Wednesday of breaking the law by attacking Syrian President Bashar Assad's forces -- a cold welcome for Secretary of State Rex Tillerson.

"We have seen very alarming actions recently with an unlawful attack against Syria," Russian Foreign Minister Sergey Lavrov said, referring to the cruise missiles President Trump was said to order, days after the U.S. accused Syria of launching a chemical attack on its own people. "We consider it of utmost importance to prevent the risks of replay of similar action in the future."

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It was an ominous start to Tillerson's visit -- the first to Russia by a Trump Cabinet official. Tillerson conceded the two world powers had "sharp differences" that have obstructed cooperation but voiced optimism that their talks could narrow those differences.

Meanwhile, Putin insisted that relations with the U.S. had only gone downhill since Trump took office in January. "The level of trust at the working level, especially at the military level, has not become better but most likely has degraded," Putin said in an interview broadcast Wednesday by state television channel Mir.

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Lavrov said Russia was trying to understand the "real intentions" of the Trump administration. He added that Moscow had lots of questions about the "very ambiguous" and "contradictory" ideas coming out of Washington.

Trump, meanwhile, told Fox Business Network that the U.S. had no plans to become more deeply involved in Syria and only did so because of last week's deadly chemical weapons attack that killed dozens. Turkey has said tests showed sarin gas was used.

"Are we going to get involved with Syria? No," Trump told "Mornings with Maria." "But if I see them using gas...we have to do something."

Trump added, "Frankly, Putin is backing a person that's truly an evil person," calling Assad "an animal."

Speaking in Moscow, Tillerson assured reporters, "We both have agreed our lines of communication shall always remain open."

It was unclear whether Putin, who once gave Tillerson an "Order of Friendship" award, would grant the visiting American an audience. Though the Kremlin had declined to say whether the leaders would meet, Putin spokesman Dmitry Peskov said Wednesday: "There is a certain likelihood."

Russia, Assad's staunchest ally, has insisted that Assad is blameless for the chemical attack and that it was actually the rebels who were responsible.

Intelligence services from several Western countries strongly dispute that claim. The health minister in Turkey, which treated many of the attack's victims and conducted autopsies on others, said Tuesday that test results conducted on victims confirmed sarin gas was used.

Tillerson, on a mission to persuade Russia to abandon Assad, issued an ultimatum to Putin before flying to Moscow: Either side with the U.S. and likeminded countries, or with Assad, Iran and the militant group Hezbollah.

"Our policy is consistent and it's formulated exclusively on the basis of international law and not under the impact of current opportunistic motives or false choice: 'You are with us or against us,"' Lavrov told Tillerson.

The Associated Press contributed to this report.