The United States and France both said Sunday that there was strong evidence to indicate that Syrian President Bashar Assad indeed used chemical weapons in deadly attacks near Damascus last week.

Shortly after the two Western powers issued their assessment, Syria announced that it had agreed to let United Nation inspectors visit the site of the chemical weapons attack, state TV reported. The inspectors are scheduled to visit the site on Tuesday.

Also on Sunday, Russia warned the U.S. against repeating past mistakes, saying that any unilateral military action in Syria would undermine efforts for peace and have a "devastating impact" on the security situation in the Middle East.

Referring to the U.S. military intervention in Iraq, the Russian Foreign Ministry said in a statement that any military action would affect joint U.S.-Russian efforts for an international peace conference on Syria.

"We once again decisively urge (the United States) not to repeat the mistakes of the past and not to allow actions that go against international law. Any unilateral military action bypassing the United Nations will ... lead to further escalation (in Syria) and will affect the already explosive situation in the Middle East in the most devastating way," the ministry said.

Syria's announcement concerning the UN inspectors came just after a senior member of the Obama administration said there was "very little doubt" that a chemical weapon was used by the Syrian regime against civilians in an incident that killed at least a hundred people last week.

The official said that the U.S. intelligence community based its assessment given to the White House on "the reported number of victims, reported symptoms of those who were killed or injured, and witness accounts." The official spoke on condition of anonymity because the official was not authorized to speak publicly.

The official also said that the White House believes the Syrian government is barring a UN investigative team immediate access to the site of a reported August 21 chemical weapons attack in the Damascus suburbs in order to give the evidence of the attack time to degrade.

French President Francois Hollande, meanwhile, said a "body of evidence" suggested that chemical weapons were used during the attacks and that Assad's regime was most likely behind it.

According to a statement Sunday from his office, Francois Hollande said "everything" leads France to believe the regime was behind the attack. His office did not elaborate.

The International aid group Doctors Without Borders said Saturday it had tallied 355 deaths from Wednesday's attack in Damascus' eastern Ghouta suburb.

U.S. President Barack Obama and top advisers are hashing out options for responding to the reported use of chemical weapons in Syria amid what Britain called "increasing signs" that the Syrian government was responsible for Wednesday's nerve gas attack on civilians in a rebel-dominated area..

Syrian authorities warned the United States on Sunday against any military action over the suspected chemical weapons attack in Syria's civil war, saying this would "create a ball of fire that will inflame the Middle East."

Secretary of State John Kerry called Syrian Foreign Minister Walid al-Moallem early Thursday and told him that the Damascus government should have allowed a full United Nations inspection of an alleged gas attack should the regime really have nothing to hide.

Kerry called "to make clear that if, as they claimed, the Syrian regime has nothing to hide, it should have allowed immediate and unimpeded access to the site rather than continuing to attack the affected area to block access and destroy evidence," a State Department official said.

"The secretary further emphasized ... that he had received full assurances from Free Syrian Army commanders that they would ensure the safety of UN investigators into the targeted areas," the official said.

Kerry, who participated remotely in a White House meeting on the potential U.S. response to the alleged use of chemical weapons in Syria, made a round of diplomatic calls on Saturday to his counterparts in Saudi Arabia, Jordan and Turkey, the official said.

"In all these calls, the secretary emphasized the importance of quickly determining the facts and underscored the seriousness and gravity of any chemical weapons use," the official said.

Syrian President Bashar Assad's government has accused the insurgents of firing the chemical weapons "as a last resort" to try to provoke foreign intervention on their side.

Meanwhile, Syrian Information Minister Omran Zoabi, in remarks released by the official news agency SANA late on Saturday night, said that any U.S.-led military action would be "no picnic".

"U.S. military intervention will create a very serious fallout and a ball of fire that will inflame the Middle East," Zoabi said. 

He also suggested that UN inspectors would not be allowed to visit the site of the alleged nerve gas attack as it was not part of a previously agreed list of locations where opposition activists say government forces used chemical weapons. Syrian authorities have denied any use of poison gas in the conflict.

Zoabi said Damascus would cooperate "significantly and transparently" with UN investigations but not allow any "inspection that will prejudice national sovereignty."

Iran, Assad's most powerful Middle East ally, also warned the United States against crossing the "red line" on Syria, saying this would have "severe consequences".

"America knows the limitation of the red line of the Syrian front and any crossing of Syria's red line will have severe consequences for the White House," Massoud Jazayeri, deputy chief of staff of Iran's armed forces, was quoted by the semi-official Fars news agency as saying.

He was responding to weekend statements by Western officials regarding the possibility of military intervention in Syria.

Syrian opposition reports that between 500 and well over 1,000 civilians were killed on Wednesday by gas in munitions fired by pro-government forces, and video footage of victims' bodies, have stoked Western demands for a robust response after two years of international inaction on Syria's conflict.

Meanwhile, Iran on Sunday warned the United States against crossing the "red line" on Syria, saying it would have "severe consequences", according to the Fars news agency.

"America knows the limitation of the red line of the Syrian front and any crossing of Syria's red line will have severe consequences for the White House," said Massoud Jazayeri, deputy chief of staff of Iran's armed forces, reacting to statements by Western officials regarding the possibility of military intervention in Syria, according to Fars.