Donald Trump has congratulated Turkish President Recep Tayyip Erdogan on his victory in Sunday's referendum that gave him sweeping new powers.

In a phone call, the US president also thanked Mr Erdogan for supporting America's missile strike on a Syrian government airbase on 7 April.

In the poll, 51.4% of Turkish voters backed the changes.

Mr Erdogan rejected criticism from international monitors who said he had been favoured by an "unequal campaign".

"Know your place," he told the observers.

The narrow victory was ruled valid by Turkey's electoral body, despite claims of irregularities by the opposition.

In a separate development, Turkey extended the state of emergency for three months. The measure, introduced after a failed coup last July, was set to expire in two days.

In a statement on Monday, the White House said President Trump discussed with his Turkish counterpart America's "action in response to the Syrian regime's use of chemical weapons on 4 April".

It said the two leaders "agreed on the importance of holding Syrian President Bashar al-Assad accountable".

They also discussed "the counter-ISIS [Islamic State campaign]", the statement added.

Relations between Washington and Ankara have recently been strained by several key issues.

One of Turkey's main grievances with the US is the policy started by the Obama administration of supporting Kurdish fighters in Syria who are fighting IS forces.

Turkey views the Syrian Kurdish People's Protection Units (YPG) as a terror group linked to Kurdish separatists waging an insurgency inside Turkey since 1984.

The two sides are also at loggerheads over Fethullah Gulen. Turkey accuses the cleric of orchestrating the failed coup.

Officially the Washington insists any decision on returning him to Turkey from the US remains a judicial rather than a political one.

Monitors: 'Unlevel playing field'

"In general, the referendum did not live up to Council of Europe standards," said Cezar Florin Preda, head of the Council of Europe delegation.

The council is a pan-European human rights body of which Turkey is a member.

The monitors also criticised a late change by electoral officials that allowed voting papers without official stamps to be counted. They said this move "removed an important safeguard and were contested by the opposition.

But the head of Turkey's electoral body, Sadi Guven, said the unstamped ballot papers had been produced by the High Electoral Board and were valid. He said a similar procedure had been used in past elections.

Turkey reacts: 'Politically motivated reports'

Addressing supporters in the presidential palace in Ankara, Mr Erdogan said that Turkey did not "see, hear or acknowledge the politically motivated reports" of the monitors.

The result, he said, ended the debate on changing the constitution and creating an executive presidency, adding that the process of implementing the reforms would now begin.

He also said the country could hold a referendum on its long-stalled EU membership bid.

Additionally, Mr Erdogan said he would approve the death penalty if it was supported in a referendum or a bill was submitted to him through parliament. This would end Turkey's EU negotiations.

Earlier, Turkey's foreign ministry said the remarks by the monitors lacked impartiality. "Saying the referendum fell below international standards is unacceptable," it said in a statement.

The main opposition Republican People's Party (CHP) has demanded a recount of 60% of the votes. Its deputy head said the result should be annulled altogether.

The pro-Kurdish Peoples' Democratic Party (HDP) also challenged the vote.