United States offers five military options to control the conflict in Syria 2307132.

Washington has so far ruled out military intervention in Syria. Its role in the conflict is currently limited to delivering humanitarian aid to Syrian refugees in neighbouring countries and providing non-lethal help to the Syrian opposition.

In an open letter to senators, Gen Dempsey analysed five military options the US military could potentially undertake in Syria:

- training, advising and assisting the opposition
- conducting limited strikes
- establishing a no-fly zone
- creating buffer zones inside Syria
- controlling Damascus' chemical arms

Gen Dempsey estimated that the first option would cost about $500m (£325m) a year, while each of the other four actions would require roughly $1bn a month.

"The decision to use force is not one that any of us takes lightly. It is no less than an act of war," he wrote in the letter.

Gen Dempsey acknowledged that the outlined actions would strengthen the opposition and put more pressure on President Bashar al-Assad, but he warned the US should learn from its previous interventions, as in Afghanistan and Iraq.

"We have learned from the past 10 years, however, that it is not enough to simply alter the balance of military power without careful consideration of what is necessary in order to preserve a functioning state," he said.

Gen Dempsey's letter was in response to questions by two members of the Senate Armed Services Committee which is considering re-appointing him for a second two-year term as chairman of the joint chiefs.

More than 90,000 people have been killed in Syria since the uprising again President Assad began in 2011, according to the UN.

A further 1.7 million have been forced to seek shelter in neighbouring countries.