(CNN) — WASHINGTON (CNN) -- United States military involvement in Syria would likely cost billions of dollars and carry a range of risks for the forces involved, Joint Chiefs of Staff Chairman Martin Dempsey said in a letter released Monday.
"I know that the decision to use force is not one that any of us takes lightly," Dempsey wrote in the letter to Sen. Carl Levin,D-Michigan, chairman of the Senate Armed Services Committee. "It is no less than an act of war."
Dempsey's letter was in response to a request by Levin and Sen. John McCain, R-Arizona, to provide his assessments of possible scenarios for future involvement in the Syrian civil war.
But it also came with a warning for a military now in a second decade at war.
"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."
Establishing a no-fly zone in Syria would cost $500 million initially, while "averaging as much as a billion dollars per month over the course of a year," Dempsey said of an operation that would limit as much as possible the aerial bombing capabilities of Syrian President Bashar al-Assad.
Dempsey said establishing a no-fly zone could result in the loss of U.S. aircraft, which would require personnel recovery forces in Syria. "It may also fail to reduce the violence or shift the momentum because the regime relies overwhelmingly on surface fires -- mortars, artillery, and missiles," he wrote.
Options to prevent the use and proliferation of chemical weapons would also include lethal force through the destruction of known stockpiles, movement interdiction, or through the physical seizure of known chemical weapons sites.
Dempsey said this option would also require a no-fly zone along with "air and missile strikes involving hundreds of aircraft, ships, submarines."
"Thousands of special operations forces and other ground forces would be needed to assault and secure critical sites," Dempsey wrote. "Costs could also average well over $1 billion per month."
It is extremely rare for the costs of such operations to be laid out in such detail, and Dempsey also noted the potential costs of less expansive actions the United State could take.
Training, advising and supporting opposition forces could require as many as several thousand troops at an estimated cost of $500 million per year initially, Dempsey said.
Options for establishing safe zones or buffer areas to allow for the training of opposition forces, as well as areas for the safe distribution of humanitarian aid, would require a limited no-fly zone to keep the areas safe from the Assad regime's aerial bombardments. U.S. ground forces would be needed to defend the safe zones, Dempsey said.
This too, could cost a great deal of money and put lives at risk Dempsey wrote.
"A limited no-fly zone coupled with U.S. ground forces would push the costs over $1 billion per month," he wrote. "Risks are similar to the no-fly zone with the added problem of regime surface fires into the zones, killing more refugees due to their concentration. The zones could also become operational bases for extremists."
Dempsey said the use of periodic and limited strikes against regime military assets would also cost "billions" depending on the duration of such operations.
The letter comes at complex time in the evolution of the Obama administration's policy on Syria. Although the administration has recently signaled its readiness to provide certain arms to vetted factions of the Syrian opposition, there has been no movement of U.S. weapons due to concerns on Capitol Hill about how the program would work.
But Rep. Mike Rogers, a Michigan Republican and the chairman of the House Intelligence Committee, signaled there may be movement on the issue in the coming days.
"The House Intelligence Committee has very strong concerns about the strength of the administration's plans in Syria and its chances for success," Rogers said in a written statement Monday. "After much discussion and review, we got a consensus that we could move forward with what the administration's plans and intentions are in Syria consistent with committee reservations."
McCain put a hold on Dempsey's nomination for a second term last week until he received greater detail from Dempsey about the various options available to intervene in Syria. It was unclear whether Dempsey's letter answered McCain's questions.