Trump called the Turkish leader on Monday shortly after international monitors delivered a harsh verdict on the referendum
on constitutional changes. They found that the opposition campaign had been restricted and the media coverage was imbalanced, and that the electoral authority had unfairly changed the rules after polls had opened.
Trump joins a short list of leaders who have openly congratulated Erdogan, including Azerbaijani President Ilham Aliyev and Saudi King Salman.
His congratulations stands in stark contrast to the more cautious tone adopted by European leaders and a statement issued by the US State Department, which acknowledged the results but warned against further repression by the Turkish government of the political opposition.
"We look to the government of Turkey to protect the fundamental rights and freedoms of all its citizens -- regardless of their vote on April 16 -- as guaranteed by the Turkish constitution and in accordance with Turkey's international commitments," the State Department said in a statement.
"Democracies gain strength through respect for diverse points of view, especially on difficult issues."
The Turkish government has been widely condemned by Western nations for its repression of opposition figures following a failed military coup last year.
A senior official with the Trump administration said the President did not raise any concerns about the referendum with Erdogan, describing that part of the phone call as a "simple congrats." The call was mainly focused on Syria, the official told CNN.
Europe sends warnings
European governments struck a more cautious note.
"The tight referendum result shows how deeply divided Turkish society is and that means a big responsibility for the Turkish leadership and for President Erdogan personally," Germany's Chancellor Angela Merkel and Foreign Minister Sigmar Gabriel said in a joint statement, calling for a "respectful dialogue" in Ankara with the opposition and all parts of Turkish society.
Erdogan's margin of victory in the referendum was razor-thin. Despite a state of emergency and a widespread crackdown on dissent, he succeeded in persuading only 51.4% of voters to back his constitutional upheaval.
The French government said it would "follow with great care" the international monitors' final report in coming weeks, particularly in relation to a reported last-minute change of rules by the electoral boards to allow ballots that had not been officially stamped. The change has raise concerns of "ballot-stuffing" -- where extra votes are cast illegally to manipulate results.
"Only the Turks can decide their political organization. But figures already published show that Turkish society is divided on this planned important reforms," it said, calling for "a free and sincere dialogue" in Turkey among all components of political and social life.
Erdogan has lashed back at the OSCE international monitors' initial findings, telling them to "know their place," Reuters reports.
He said that Turkey did not "see, hear or acknowledge" the observer mission's reports and accused some European countries of showing more opposition to the constitutional changes than Turkey's own opposition parties.
Relations between Turkey and Europe's powerhouse nations have come under strain over Erdogan's comments that he would seek the restoration of the death penalty -- a move that would sink Turkey's long-stalled bid to join the European Union.
"We have said this over and over in my speeches. As I said this will come before the parliament and if it is passed from the parliament, I would approve this. I would confirm. Why? Because we do not have the authority to forgive the murders of our martyrs," Erdogan said at a rally Monday.