Fifty-one U.S. State Department officials signed an internal memo calling on President Barack Obama's administration to take military action against Syria's Bashar Assad in a bid to force regime change in the country, a suggestion that has been repeatedly made by Ankara for a long time.

Signed by officials working on U.S. Syria policy, the "dissent channel cable" makes repeated references to "targeted military strikes" on the Assad regime, according to multiple reports published on Thursday.

"Failure to stem Assad's flagrant abuses will only bolster the ideological appeal of groups such as DAESH, even as they endure tactical setbacks on the battlefield," the cable reads as cited in The New York Times.

"The moral rationale for taking steps to end the deaths and suffering in Syria, after five years of brutal war, is evident and unquestionable," it said. "The status quo in Syria will continue to present increasingly dire, if not disastrous, humanitarian, diplomatic and terrorism-related challenges."

Obama has refrained from taking military action, supposedly due to concerns that it could ignite a conflict with Russia and lead to greater instability in the war-ravaged nation.

Assad has been buoyed by his allies, with Russia carrying out airstrikes in the country that have assisted his advances since September and, Iranian forces and Hezbollah fighting alongside Assad's Syrian Arab Army on the ground.

In a sign of Assad's growing confidence, he vowed last week to retake "every inch" of Syrian territory from his enemies.

The Wall Street Journal said the complaints being lodged by State Department officials are not unusual, but the number of signatories opposing U.S. policy is, according to current and former State Department officials.

"It's embarrassing for the administration to have so many rank-and-file members break on Syria," a former State Department official who worked on Middle East policy said, according to the Wall Street Journal.

Presumptive Democratic presidential nominee Hillary Clinton has vowed to be more hawkish on Syria than Obama and the document may be an attempt to inform the policy of the next president, the Journal said.

ERDOĞAN ALSO UNHAPPY WITH OBAMA'S SYRIA POLICY

Amid astonishment due to the dissent in the U.S. State Department, President Recep Tayyip Erdoğan recently shared his frustration with Obama's Syria policy.

"Although bilateral relations with the U.S. had a good start when U.S. President Barack Obama assumed office in 2008, Turkey's expectations could not be met with the passing of time," Erdoğan said last week.

Stressing that Ankara's expectations were not met in terms of partnership in the area of economy, Erdoğan said: "Expectations were not met in foreign policy either. Obama had said that he would bring a solution to the Iraqi crisis and U.S. troops would completely withdraw from there. This did happen, but they were transferred to Afghanistan."

Washington's support of the People's Protection Units (YPG) has troubled Turkish-U.S. relations to a great extent. Special Presidential Envoy for the Global Coalition to Counter ISIL (DAESH) Brett McGurk paid a visit to Democratic Union Party- (PYD) controlled Kobani and held talks early in February. Photos of McGurk's visit to Kobani surfaced on social media.

McGurk was seen meeting with former PKK fighters in Syria. Polat Can, the spokesman and one of the founders of the YPG, shared a photo on his Twitter account showing him presenting a plaque to McGurk.

Some U.S. special forces soldiers were photographed with YPG insignias on their sleaves. Ankara harshly criticized the U.S. for the photos, saying it could damage relations.

The two countries differ on how the Manbij operation should have been carried out and what will happen after it is completed.

Even though the U.S. administration insists that the YPG is not the driving force in the Manbij operation, Ankara remains suspicious and asserts that it does not welcome the group in Manbij once the operations have been concluded.

Some 3,000 Arab militants are taking part in the offensive, backed by around 500 Kurdish militia members, U.S. Central Command spokesman Colonel Patrick Ryder said previously, adding that U.S. special forces were working "at the command and control level" in the operation.

Erdoğan said the U.S. had assured Turkey that mostly Arab forces are conducting the Manbij operation instead of Kurdish members of the YPG. "The main forces are Arab militants. That is what we were told," he said.