The top US military officer warned senators on Monday that taking military action to stop the bloodshed in Syria was likely to escalate quickly and result in "unintended consequences", representing the most explicit uniformed opposition to deeper involvement in another war in the Middle East.

Alluding to the costly, bloody occupations of Iraq and Afghanistan, General Martin Dempsey, chairman of the joint chiefs of staff, said that once the US got involved militarily in Syria, where dictator Bashar al-Assad has slaughtered an estimated 90,000 people, "deeper involvement is hard to avoid".

"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," Dempsey wrote to senators John McCain and Carl Levin on Monday. "We must anticipate and be prepared for the unintended consequences of our action."

Dempsey's letter came after McCain announced he would block the general's reappointment to chair the joint chiefs of staff, the most senior position in the US military, until Dempsey provided the Senate with his assessment of the merits of US military action in Syria.