Russia has condemned a US plan to target Isis (now known as Islamic State) militants in Iraq and Syria, saying that it would consider air strikes in the latter country an "act of aggression", if carried out without a UN mandate.
Moscow - Syria's main international ally - claimed that for the operation to be legal under international law, the regime of President Bashar al-Assad should consent to it.
"The US president has spoken directly about the possibility of strikes by the US armed forces against [Islamic State] positions in Syria without the consent of the legitimate government," Foreign Ministry spokesman Alexander Lukashevich said.
"This step, in the absence of a UN Security Council decision, would be an act of aggression, a gross violation of international law."
The Russian Foreign ministry tweeted:
To be legitimised by the UN a military operation has to be approved by the Security Council, where Russia has veto power.
The Kremlin had already fiercely opposed US direct involvement in Syria last year, as Washington was considering punitive strikes after the regime used chemical weapons in a deadly attack against a rebel-held area of Damascus.
Moscow's renewed hostility comes after US president Obama announced he has authorised air strikes against IS inside Syria in a prime-time address to the nation from the White House.
"If you threaten America, you will find no safe haven," Obama said.
Damascus also warned Washington against launching air strikes on Syrian soil.
"Any action of any kind without the consent of the Syrian government would be an attack on Syria," said Ali Haider, Syrian Minister for Reconciliation.
Obama has ruled out any cooperation with Assad as he saying the Syrian leader will "never regain the legitimacy" he has lost after atrocities carried out by his regime during the three-year civil war.
The US has already launched limited air strikes against IS in Iraq at the request of former Iraqi Prime Minister Nouri al-Maliki.