WASHINGTON - As the West prepares for possible strikes against Syria, the US Navy has deployed a fifth destroyer to the eastern Mediterranean while Damascus ally Russia will be sending an anti-submarine ship and a missile cruiser to the area "over the next few days."

President Barack Obama said Wednesday he had not yet signed off on a plan to attack Syria, but action appeared likely after Washington stopped seeking a UN mandate.

And British government said it would still be permitted under international law to take military action against Syria on even if such a move is blocked by the UN Security Council, according to legal advice published Thursday.

The government said the advice states that the legal basis for any such action would be "humanitarian intervention" and "if action in the Security Council is blocked, the UK would still be permitted under international law to take exceptional measures".

At home, Britain's Labour opposition said Thursday that it was set to vote against the principle of military intervention in Syria. "We have been having increasing doubts about the opaque nature of the government's motion. It does not mention anything about compelling evidence," a party source said.

UN chief Ban Ki-moon said Thursday his investigating team would report back this week on suspected chemical weapons attacks in Syria. He said the UN chemical weapons experts – who headed out to one of the attack sites near Damascus for the third day of inspections – would leave Syria by Saturday and report to him immediately. Ban appealed to divided international powers to work together to head off military action in Syria, where the UN says 100,000 people have been killed and several million made homeless since the conflict erupted in March 2011. "Diplomacy should be given a chance ... peace (should) be given a chance," Ban said, adding that he had spoken to Obama about the situation.

With any US-led missile strike unlikely to have UN Security Council backing, key Damascus allies Russia and Iran again warned against any Western intervention, saying it could set off a wider regional conflict. Syria's nervous neighbours have already stepped up preparations for conflict, with Israel authorising a partial call-up of arm reservists while Turkey put its forces on heightened "vigilance".

Russia is also reportedly sending warships to the Mediterranean, while Britain said it was sending fighter jets to the strategic island of Cyprus.

The USS Stout, a guided missile destroyer, is "in the Mediterranean, heading and moving east" to relieve the USS Mahan, said a US official, who said both ships might remain in place for the time being.

A source in the Russian General Staff told Interfax, "The well-known situation shaping up in the eastern Mediterranean called for certain corrections to the make-up of the naval forces. A large anti-submarine ship of the Northern Fleet will join them (the existing naval forces) over the next few days.

"Later it will be joined by the Moskva, a rocket cruiser of the Black Sea Fleet which is now wrapping up its tasks in the northern Atlantic and will soon begin a Transatlantic voyage towards the Strait of Gibraltar." In addition, a rocket cruiser of the Pacific Fleet, the Varyag, will join the Russian naval forces in the Mediterranean this autumn by replacing a large anti-submarine ship.

However, the state-run RIA Novosti news agency cited a high-ranking representative of the naval command who said the changes to the country's forces in the region were not linked to the current tensions over Syria and called them "a planned rotation."

Obama, who has warned that the use of chemical weapons in Syria would cross a US "red line", said Washington had definitively concluded that the Assad regime was to blame for last week's attack. A senior White House official told AFP that the administration will brief senior US lawmakers on Thursday about classified intelligence about the chemical attack.

Asked how close he was to ordering a US strike, expected to start with cruise missile raids, Obama told PBS NewsHour: "I have not made a decision." But he warned that US action would be designed to send a "shot across the bow" to convince Syria it had "better not do it again."

In Damascus, soldiers were being pulled back from their command posts and tougher security controls were in place at roadblocks and hospitals. A Syrian security official said the army was preparing for the worst.

Syrian President Bashar al-Assad, whose army has been gaining ground against the rebels in recent months, vowed his country would emerge the victor in any confrontation with the US.

"Since the start of the crisis... we have waited for our true enemy to reveal itself," Lebanon's Al-Akhbar newspaper quoted Assad as telling Syrian officials. "It's a historic confrontation from which we will emerge victorious," said Assad, whose regime has blamed "terrorist rebels" for the gas attack.