President Barack Obama initially viewed Erdogan as a possible model for a new generation of Muslim leaders and travelled to Turkey early in his first year in office.
The pro-Kurdish opposition Peoples' Democratic Party said it presented complaints about unstamped ballots affecting 3 million voters, more than twice the margin of Erdogan's victory.
EU Commission spokesman Margaritis Schinas called on Turkish authorities "to launch transparent investigations into these alleged irregularities found by the observers".
The "Yes" camp won 51.41 percent in Sunday's referendum.
On Sunday, the YSK suddenly announced that ballots that had not been verified as genuine by election officials would be allowed, which led to a protest by the main opposition Republican People's Party (CHP), which said it would contest the result.
Global election monitors also delivered a scathing verdict Monday on the conduct of the referendum.
"They have a right to have elections and their people participated in that", Spicer said.
Erdogan's victory was far tighter than expected, emerging only after several nail-biting hours late Sunday which saw the "No" result dramatically catch up.
North Korean state media warns of 'super-mighty pre-emptive strike'
One propaganda film previous year featured animation of a North Korean, nuclear-tipped ICBM striking New York City. Almost a month ago, the totalitarian nation released a fiery video with scenes including troops blowing up a U.S.
"Now Erdogan will have to rule the part of Turkey most open to the world, with the highest cultural production, export capacity, tourism revenue and industrial output, with a constitution approved by its most introvert part", he wrote.
Supporters of the "no" vote, chant slogans during a protest against the referendum outcome, on the Aegean Sea city of Izmir, Turkey, Tuesday, April 18, 2017.
If Erdogan presses ahead with reinstating the death penalty, the AKP will need to either pass a bill through parliament, for which it does not have the votes alone, or hold another referendum, which he could swing with the backing of the nationalist MHP party, which has supported the idea in the past.
French President Francois Hollande also urged dialogue between the political factions.
There was talk Erdogan would call an election so his powers could take effect. "As a result of the YSK decision, it was impossible to prove whether ballot papers were brought from the outside, because the only proof is the lack of a stamp", the TBB also stated.
Erdogan reaffirmed he would now hold talks on reinstating capital punishment - a move that would automatically end Turkey's European Union bid - and would hold a referendum if it did not get enough votes in parliament to become law.
Saudi Arabia, meanwhile, praised the "success" of a referendum.
The new referendum would allow Erdogan, who has already enacted sweeping reforms since a 2016 uprising that almost ended in a coup, "to issue decrees, declare emergency rule, appoint ministers and top state officials and dissolve parliament", according to Reuters. The cabinet said it hoped the vote would contribute to "more development success across the country".
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