WASHINGTON - Despite some military defections, Syrian President Bashar Assad's inner circle remains cohesive and the 16-month conflict with rebels is likely to be a drawn-out struggle, senior US intelligence officials said on Tuesday.That assessment appears to dash any US hopes that Assad, whose ouster Washington has called for, will fall soon of his own accord. The Obama administration has declined to intervene militarily in Syria, citing the lack of international backing and the country's sectarian divisions.Assad's forces, opposition engaged in 'see-saw battle'US intelligence officials said defections and desertions from Assad's forces had mainly been "low to mid-level ranks" and had included relatively few officers. They did not provide specific numbers.Assad's forces have been engaged in a "see-saw battle" with opposition forces in which the military strikes hard, then the rebels change tactics and gain momentum, followed by the military forces stepping up again, the US officials said."Our overall assessment in terms of the fighting would be that we are still seeing the military regime forces fairly cohesive, they've learned some lessons over the last year and a half about how to deal with this kind of insurgency," an intelligence official said.The insurgency is also getting stronger, which sets the stage for a protracted conflict, the official said."Both sides seem to be girding for a long struggle. Our sense is that the regime still believes it can ultimately prevail or at least appears determined to try to prevail and the opposition at the same time seems to be preparing for a long fight," the intelligence official said.Russia has provided Syrian forces with advanced air defense systems as well as attack helicopters, while Iran and the Lebanese Hezbollah have provided lethal and non-lethal aid such as small arms, communications equipment, and riot-control gear, intelligence officials said.Weapons crossing the border to the rebels are mostly small arms such as AK-47 automatic weapons, and some anti-tank guided munitions and rocket-propelled grenades from sympathizers in Saudi Arabia and Qatar, officials said.US intelligence agencies see Assad's government as being on a downward slide."It is very difficult for the regime to completely suppress the opposition and I do think the overall trendline for the regime is downward," an intelligence official said.But officials acknowledge uncertainties, and are considering various scenarios for Syria's future.The longer the violence goes on, the greater the chance of a sectarian civil war similar to Iraq's in 2005-2007, and in that scenario, Assad could rule over a rump government or a part of Syria, having lost control of other parts of the country, the official said.Assad comes from the minority Alawite sect while the majority of the population in Syria is Sunni Muslim.Another scenario could be that some Sunni elites or Sunni elites and some Alawites see the way out "is to kill Assad or to move him aside" and try to arrange some kind of transition, the intelligence official said.Another possible scenario would be Alawites fighting very hard and their numbers reduced over time, leading to the collapse of Assad's government and yielding disorder without anybody in full control."There definitely are a number of outcomes here that I think are possible and right now I would not really make a hard call of one over the other," the intelligence official said.White House spokesman Jay Carney said on Tuesday that Assad "has been slowly, too slowly, losing his grip over his country. The process, because of his refusal to step aside, has been horrific and has exacted a terrible toll on the Syrian people."But US intelligence agencies, watching closely for cracks in Assad's inner circle, do not see them so far."The regime inner circle and those at the next level still seem to be holding fairly firm in support of the regime and Assad," one intelligence official said on condition of anonymity.Assad said on Tuesday that Syria was in a "state of war" and snubbed those calling for him to step aside, saying the West "takes and never gives and this has been proven at every stage."Despite the deterioration in Syria, so far there has been no sign of an appetite for Western intervention like that by NATO against Libya's Muammar Gaddafi last year. On Tuesday, the Western alliance called the shooting-down of a Turkish warplane by Syrian forces last week "unacceptable" but stopped short of threatening retaliation.