Currently, there are a number of communication lines between Russian and USA forces, including one that runs into the US -led coalition's main operations center in Qatar and another that connects the commander of US forces in Iraq and Syria, Lt. Gen. Stephen Townsend, to his Russian counterpart in Syria.
The Pentagon says the USA has seen chemical weapons activity at a Syrian air base that was used for an April sarin gas attack.
He said the United States would not fire "unless they are the enemy, unless they are Isis".
Late Monday, the White House issued a warning to Syrian President Bashar Assad based on "potential" evidence that Syria was making preparations to carry out another chemical attack.
"If we are responsible for any civilian casualties we come forth and admit it", he said.
North Atlantic Treaty Organisation ally Turkey, which views the YPG as a threat for its links to the outlawed Kurdistan Workers' Party (PKK), has said Mattis assured it in a letter that the United States would eventually take back the weapons it was giving them once ISIL was defeated.
Mattis also said that as the fight against ISIS grows more complex and moves into the Euphrates River Valley, it will require more deconfliction with the Russians.
It depends, he said, on the battle and what weapons the Kurdish fighters need.
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"We are very clear the use of chemical weapons by the Syrian regime is absolutely abhorrent and the last time the United States took action to deal with the aircraft and airbase from which these chemical weapons were used we fully supported their strike", Fallon told the BBC. But Mattis also noted that YPG militants were well-armed even before the USA last month made a decision to offer more specialised equipment for its urban assault on ISIL-held city of Raqqa.
The UK would support any United States attack on Assad so long as it was proportionate, legal and necessary, Fallon added.
North Atlantic Treaty Organisation ally Turkey, which views the YPG as a threat, has said Mattis assured it in a letter that the United States would eventually take back the weapons it was giving them once Islamic State was defeated.
Turkish officials late last week said Mattis had reassured them by letter that arms given to the Syrian Kurds would be taken back and that the US would provide Turkey with a regular list of arms give to the fighters.
Turkey sees the YPG as an extension of the outlawed Kurdish PKK, which has been waging an insurgency in the country's southeast since the mid-1980s. The Turkish government is concerned that the US arms and equipment given to the YPG will end up in the hands of the PKK.
On Monday, while Mattis didn't directly confirm the letter's existence, he did say the Pentagon would "recover that sort of gear" after it's no longer needed. Around 100 inmates were imprisoned in that jail, said the Observatory.
Handcuffs are pictured inside a prison, which, according to SDF fighters, belonged to Islamic State militants in Manbij, Aleppo Governorate, Syria, August 17.