Several U.S. officials said that while the White House is prepared to hear the diplomats’ dissenting viewpoint, it is not expected to spur any changes in President Barack Obama’s approach to Syria in his final seven months in office.

One senior official said that the test for whether these proposals for more aggressive action are given high-level consideration will be whether they “fall in line with our contention that there is no military solution to the conflict in Syria.”

The document - sent through the State Department’s “dissent channel,” a conduit for voicing contrary opinions meant to be confidential - underscored long-standing divisions and frustrations among Obama’s aides over his response to Syria’s five-year-old civil war.

Obama's Syria policy has been predicated on the goal of avoiding deeper military entanglements in the chaotic Middle East, and has been widely criticized as hesitant and risk-averse. Obama's limited intervention has focused on fighting the Islamic State militant group that controls a swathe of Syria and Iraq and which has inspired attacks on U.S. soil.

A draft of the cable, signed by 51 State Department officers, calls for "targeted military strikes" against Assad’s government - something Obama has long opposed - to stop its persistent violations of a ceasefire with U.S.-backed anti-government rebels that is largely ignored by Syria and its Russian supporters.

The document, initially crafted in secret by a small group before their State Department bosses were made aware, was intended to "spark internal discourse" toward a policy shift but was not meant to be made public, according to a person familiar with the matter.

The signatories, mostly rank-and-file diplomats who had worked on Syria policy, may have put their careers at risk, current and former officials said. But State Department spokesman John Kirby insisted there would be no reprisals.

Obama’s critics quickly seized upon the letter, which also calls for a political transition that would usher Assad out.

“Even President Obama's own State Department believes the administration's Syria policy is failing,” said Ed Royce, Republican chairman of the House of Representatives Foreign Affairs committee. “Iran, Russia and Assad call the shots in Syria, ignoring the ceasefire and allowing Assad to continue war crimes against his own people."