WASHINGTON — The top American diplomat on the ground in northern Syria has criticized the Trump administration for not trying harder to prevent Turkey’s military offensive there last month — and said Turkish-backed militia fighters committed “war crimes and ethnic cleansing.’’

In a searing internal memo, the diplomat, William V. Roebuck, raised the question of whether tougher US diplomacy, blunter threats of economic sanctions, and increased military patrols could have deterred Turkey from attacking. Similar measures had dissuaded Turkish military action before.

“It’s a tough call, and the answer is probably not,’’ Roebuck wrote in the 3,200-word memo. “But we won’t know because we didn’t try.’’ He did note several reasons the Turks might not have been deterred: the small US military presence at two border outposts, Turkey’s decadeslong standing as a NATO ally, and its formidable army massing at the Syrian frontier.

In an unusually blunt critique, Roebuck said the political and military turmoil upended the administration’s policy in northern Syria. The administration’s actions left Syrian Kurdish allies abandoned, resulted in ceding territory the Kurds had controlled to Syria, Turkey, and Russia, and opened the door for a possible Islamic State resurgence.

While he described the events as a “sideshow’’ to the bloody, yearslong upheaval in Syria, he said that “it is a catastrophic sideshow and it is to a significant degree of our making.’’

Roebuck, a respected 27-year diplomat and former US ambassador to Bahrain, sent the unclassified memo Oct. 31 to his boss, James F. Jeffrey, the State Department’s special envoy on Syria policy, and to about four dozen State Department, White House, and Pentagon officials who work on Syria issues. Roebuck is Jeffrey’s deputy.

The New York Times obtained a copy of the memo from someone who said it was important to make Roebuck’s assessment public. Roebuck declined to comment Thursday.

Roebuck’s memo appears to be the first formal expression of dissent on Syria from a Trump administration official to be made public. Pentagon officials were alarmed by the sudden shift in Syria policy, but top officials never made their views public.

For nearly two years, Roebuck has worked on the ground in northern Syria with Syrian Kurdish and Arab military and civilian officials who make up what is called the Syrian Democratic Forces. Roebuck has been an important interlocutor with Mazlum Kobani, the Syrian Kurdish military commander whose fighters have worked closely with American Special Operations forces to combat the Islamic State.

Roebuck’s memo comes at a tumultuous time on the ground in northern Syria and at a delicate moment for the administration’s Syria policy. Jeffrey is scheduled to travel to Ankara and Istanbul for meetings Friday and Saturday with senior Turkish officials and members of the Syrian opposition to the government of President Bashar Assad of Syria.

The memo also came two weeks after Vice President Mike Pence agreed to a deal with President Recep Tayyip Erdogan of Turkey that accepted a Turkish military presence in a broad part of northern Syria in exchange for the promise of a five-day cease-fire, completing an abrupt reversal of US policy in the Syrian conflict. Pence hailed the agreement as a diplomatic victory for President Trump, calling it a “solution we believe will save lives.’’