Monitors have said the vote took place on an "unlevel playing field"
US President Donald Trump has called Turkish President Recep Tayyip Erdogan on the recent Turkish referendum result.
The call comes amid concerns raised by international monitoring groups and local opposition over the vote, which will grant Mr Erdogan with sweeping new powers.
The referendum - which came six months after a military coup failed to unseat President Erdogan - narrowly passed over the weekend, with 51% of voters supporting the vote.
The president's role will now be dramatically expanded, changing from a mostly ceremonial role to a nearly all-powerful position as head of government, head of state and head of the ruling party.
In a statement, the White House said Mr Trump congratulated Mr Erdogan during a phone call in which they also discussed the recent chemical attack in Syria and the fight against Islamic State.
The White House confirms that Trump congratulated Erdogan on Turkey's referendum. pic.twitter.com/0lYC6mMNmM— Philip Bump (@pbump) April 18, 2017
Mr Trump's congratulations stand in stark contrast to the cautious response from European leaders.
International monitors have said the vote took place on an "unlevel playing field and the two sides of the campaign did not have equal opportunities".
The ODIHR concluded that "fundamental freedoms essential to a genuinely democratic process were curtailed".
The Turkish leader dismissed the concerns of monitors, telling them to 'know your place'.
The European Commission noted they were waiting for information on "alleged irregularities" in the referendum, adding: "The constitutional amendments, and especially their practical implementation, will be assessed in light of Turkey's obligations as a European Union candidate country and as a member of the Council of Europe."
Antonio Tajani, president of the European Parliament, voiced concerns over Mr Erdogan's suggestion that he could hold a referendum on reintroducing capital punishment in Turkey.
In remarks translated by BBC, Austrian Foreign Minister Sebastian Kurz said the referendum result was a "clear signal against the European Union".
Turkey’s main opposition party, meanwhile, has pledged to contest the referendum result, suggesting it would take the appeal to European courts if necessary.
The People's Republican Party (CHP) yesterday raised several concerns about the vote - claiming unstamped ballots were counted and that some counts took place in private.