Washington: US President Barack Obama has pledged a "relentless" war against the jihadist Islamic State, raising crucial questions about US plans to aid rebel groups and carry out air strikes in Syria.
-- How is the Syrian regime likely to respond to US air strikes and training of rebels?
Syria warned against US air strikes on its territory after Obama`s speech, with National Reconciliation Minister Ali Haidar saying: "Any action of any kind without the consent of the Syrian government would be an attack on Syria."
But it is unclear whether Syrian forces have the will or the ability to attack US aircraft carrying out strikes.
"They will huff and puff and complain, but militarily they cannot do anything," said Riad Kahwaji, head of the Institute for Near East and Gulf Military Affairs.
The regime now has little presence in the provinces most heavily-dominated by the Islamic State -- Raqa and Deir Ezzor -- preventing ground-to-air attacks on US forces.
And "any of its ageing MiGs or Sukhois (jets)... will be shot down miles away," Kahwaji said.
-- Which groups could receive US equipment and training?
The US faces the difficulty of finding moderate and credible partners among Syria`s deeply fractured rebel forces, where conservative battalions have gained increasing strength.
It is already providing weapons to groups including the Hazm movement, which was established in 2014 with a secularist outlook and is believed to number about 15,000 fighters.
Washington could also provide training and weapons to some brigades in the Syrian Revolutionaries Front, which groups a number of battalions with a predominantly secular outlook, including some that have reportedly already received US arms.
Finally, the rebel groups operating under the umbrella of the opposition National Coalition`s Supreme Military Council are likely to be key US partners in the fight against IS.
Washington recognises the National Coalition as the official representative of the Syrian opposition and the group has regularly called for more Western aid and training.
Their forces numbers in the tens of thousands, though their ranks have been thinned by the growing influence of jihadists like IS and Al-Qaeda`s Syrian affiliate Al-Nusra Front and their presence on the ground is thin compared to other coalitions.
Washington is unlikely to offer support, however, to the strongest of the non-jihadist rebel forces in Syria -- the Islamic Front coalition, which espouses a conservative religious vision.-- What weapons and training could the rebels receive?
Louay Muqdad, a former spokesman for the rebel coalition known as a the Free Syrian Army, said training provided to opposition fighters so far had been small-scale and mostly on light weaponry.
To confront the Islamic State, opposition fighters would need a major influx of weapons, both light and medium, to sustain "street battles" against the group.
He said rebels would also need anti-tank missiles capable of taking out the tanks and other heavy military materiel that IS captured when it overran army bases in Iraq and Syria.-- How would air strikes and aid to rebels affect the fight against the Islamic State?
Syria rebel forces have already shown their ability to take on the Islamic State, pushing it out of large parts of opposition-held territory after a backlash against the group began in January.
But IS was able to retake much of its lost ground in recent months, bolstered by new recruits and heavy weaponry captured in Iraq.
Experts and rebel fighters say they would not be effective unless they receive heavier weapons and even a no-fly zone, to prevent the regime from striking opposition fighters while they tackle IS.-- What effect will the US plan have on the battle against President Bashar al-Assad`s regime?
Ideally for the West, arming "moderate" rebels to take on jihadists would have the additional benefit of better enabling them to fight Assad`s government, which the West opposes.
But it is far from clear whether that will be the outcome, and experts warn that weakening IS could also play into the regime`s hands by removing a key opponent and freeing its forces to attack other opposition groups.
Rebels also say the weapons they are likely to receive to tackle IS are not what they need to fight the regime.
They have long sought weapons to take out regime aircraft, but those are not likely to provided for the battle against IS, which has no proven aerial capacity.