U.S. Secretary of State Rex Tillerson gestures as he delivers a joint statement with Ukrainian President Petro Poroshenko in Kiev Thomson Reuters WASHINGTON (Reuters) - Forty former U.S. diplomats and national security officials urged Secretary of State Rex Tillerson not to eliminate the State Department office that handles refugees in a letter seen by Reuters on Monday.

The letter argued that a Trump administration proposal to transfer the responsibilities of the Bureau of Population, Refugees and Migration (PRM) to other agencies would undercut U.S. diplomatic leverage in grappling with foreign crises.

Among the retired officials who signed the letter were former Deputy Secretary of State William Burns, former Under Secretaries of State for Political Affairs Nicholas Burns and Wendy Sherman and former Middle East peace envoy Dennis Ross.

"Whether in Iraq, Afghanistan, Kenya, Turkey or South Sudan, the Department of State’s efforts to address humanitarian crises must include the tightest coordination of diplomatic engagement and emergency assistance," the letter, which was also signed by 18 executives with non-governmental aid agencies, said.

"We are convinced that the elimination of PRM's assistance functions would have profound and negative implications for the Secretary of State's capacity to influence policy issues of key concern to the United States," the letter said.

A memo to the Office of Management and Budget seen by Reuters proposes giving PRM's responsibility for the U.S. Refugee Admission Program, which helps resettle refugees in the United States, to the Department of Homeland Security.

Syrian Refugees seen at Lebanon's border region of Arsal Thomson Reuters

The memo, which was first reported by CNN on June 30 and which a former official said was prepared by the White House's domestic policy council, proposes transferring its other responsibilities elsewhere and eliminating the bureau.

The memo and the letter from the former officials appear to be part of a wider struggle over refugee and immigration policy.

This includes executive orders signed by President Donald Trump and challenged in court temporarily banning most refugees from entering as well as most visits by citizens of selected Muslim-majority nations, an internal debate over the cost of resettling refugees and a crackdown on illegal immigration.

The State Department had no immediate comment on the letter.

The issue may come up at a Senate Foreign Relations Committee hearing at 5 p.m. (2100 GMT) on Monday where Deputy Secretary of State John Sullivan is scheduled to testify about the State Department's reorganization plans.

(Reporting by Arshad Mohammed; Editing by Tom Brown)