The U.S. has urged the Turkey's government to protect fundamental rights and freedoms as Turkish officials work to resolve the contested results of a referendum on increasing the president's power, AP reported quoting U.S. State Department spokesman Mark Toner.

He pointed to concerns raised by the Organization for Security and Cooperation in Europe (OSCE), which monitored the Turkish referendum Sunday. He noted Monday there were irregularities both on voting day and during the campaign that led to an "uneven playing field" for the "no" side.

The OSCE has criticized a Turkish electoral board decision to allow ballots that did not bear official stamps, saying that undermined efforts to fight fraud.

According to preliminary results, the Turkish President Recep Tayyip Erdogan-led “Yes” campaign has won Sunday’s referendum in Turkey by garnering 51.2 percent of the votes.

Eighteen constitutional amendments were put to the vote on Sunday, and according to which Turkey is to transition from a parliamentary to a presidential system of government.

Turkish opposition and international analysts stress, however, that with these amendments, Erdogan is gaining the sole right to govern Turkey, and that this is perilous for democracy and freedoms in the country.

The president has already announced his plans for reinstating the death penalty in Turkey.