"I've told our American partners repeatedly that this is a big mistake. Just like they received approval from the Iraqi government, they should have obtained approval from Damascus or come to the UN Security Council," he said.
"The fact that they went there illegally reflects, firstly, the arrogant stance that Syrian President Assad heads an illegitimate regime," Lavrov said in remarks published on Thursday.
"And, secondly, in my opinion, the desire to keep their hands untied and be able to use the coalition to attack not only terrorist positions, but perhaps also the regime's forces later on in order to overthrow it, as it happened in Libya."
Lavrov said Russia is "the only country engaged in anti-terrorism activities in Syria legally."
Russian Deputy Foreign Minister Sergei Ryabkov also said Friday that the deployment of US Special Forces to Syria without coordination with Damascus violates Syria's sovereignty.
Since September 2014, the US and some of its allies have been carrying out airstrikes against purported Daesh positions inside Syria.
The Syrian government has charged that the airstrikes had targeted the country's infrastructure in many instances and done little to stop the advances of terrorists.
US 'aggression'
On Thursday, Damascus denounced the arrival of 150 US special forces in Syria, calling it a blatant violation of the country's sovereignty and an aggression.
A statement issued by the Foreign Ministry said the troops arrived in the town of Rumeilan in Syria's predominantly Kurdish province of Hasakeh in the north.
President Barack Obama announced this week the deployment of 250 more troops to Syria, which would bring the number of US Special Forces in the war-torn country to 300.
The Syrian statement said the deployment of the troops is a "rejected and illegitimate intervention that was done without the Syrian government's consent."
Russia, meanwhile, said it wants detailed information about the US plan, including the precise purpose of the deployment.
“We would like to comprehend what this is all about; whether this is a one-time action, who these people are and where they will be stationed,” Foreign Ministry spokeswoman Maria Zakharova said in Moscow.
"What they are dispatched there for and whether this is part of a program or a plan," she added.
A US State Department spokesman said earlier in the week that Obama’s promise not to put US boots on the ground in Syria was never a promise; Press TV reported.
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