Ignoring concerns raised by international monitoring groups, President Donald Trump has called Turkish President Recep Tayyip Erdogan to congratulate him on the country's contested referendum greatly increasing presidential powers.

He made the call despite protests from opposition parties and international monitoring groups, including Mr Trump's own State Department, about voting irregularities during the referendum.

Critics argue the reforms in Turkey will hand extensive power to a man with an increasingly autocratic bent and leave few checks and balances in place.

Protests continue against Turkish referendum result

Under the new system, the president will be able to appoint ministers and senior government officials, issue decrees and declare states of emergency.

Mr Erdogan has also said he would approve the death penalty if it was supported in a referendum or a bill submitted to him through parliament, which would end Turkey's aspirations to join the EU.

Mr Trump has been criticised for congratulating Mr Erdogan, who is seen as taking Turkey in an increasingly authoritarian direction.

Evan McMullin, a former Republican who ran as an independent rival to Mr Trump in the 2016 election, wrote on Twitter: "An American president should never support a foreign dictator's power grab. A simple gesture like this can weaken liberty here and abroad."

Opposition parties have complained of a series of irregularities, including an electoral board decision to accept ballots that did not bear official stamps, as required by Turkish law.

International monitors from the Organization for Security and Cooperation in Europe (OSCE) said the move undermined safeguards against fraud.

The White House said Mr Trump and Mr Erdogan discussed the US missile strike on a Syrian regime airfield in response to a chemical attack that killed dozens, with Mr Trump thanking his Turkish counterpart for supporting the strike.

Earlier, US State Department spokesman Mark Toner echoed the concerns raised by the OSCE, pointing to "observed irregularities" on voting day and "an uneven playing field" during the campaign.

Mr Erdogan, meanwhile, called the referendum "the most democratic election... ever seen in any Western country" and admonished the OSCE monitors to "know your place."

In pictures: Turkey coup attempt Show all 17 1 /17 In pictures: Turkey coup attempt In pictures: Turkey coup attempt Turkish President Erdogan attends the funeral service for victims of the thwarted coup in Istanbul at Fatih mosque on July 17, 2016 in Istanbul, Turkey Burak Kara/Getty Images In pictures: Turkey coup attempt Soldiers involved in the coup attempt surrender on Bosphorus bridge with their hands raised in Istanbul on 16 July, 2016 Gokhan Tan/Getty In pictures: Turkey coup attempt A civilian beats a soldier after troops involved in the coup surrendered on the Bosphorus Bridge in Istanbul, Turkey, 16 July, 2016 REUTERS/Murad Sezer In pictures: Turkey coup attempt Surrendered Turkish soldiers who were involved in the coup are beaten by a civilian Reuters In pictures: Turkey coup attempt Soliders involved in the coup attempt surrender on Bosphorus bridge Getty In pictures: Turkey coup attempt Supporters of Turkish President Recep Tayyip Erdogan wave flags as they capture a Turkish Army vehicle Getty In pictures: Turkey coup attempt People pose near a tank after troops involved in the coup surrendered on the Bosphorus Bridge in Istanbul, Turkey, 16 July, 2016 Reuters In pictures: Turkey coup attempt Turkish soldiers block Istanbul's Bosphorus Brigde Getty In pictures: Turkey coup attempt A Turkish military stands guard near the Taksim Square in Istanbul Reuters In pictures: Turkey coup attempt Pierre Crom/Twitter In pictures: Turkey coup attempt Turkish soldiers secure the area as supporters of Recep Tayyip Erdogan protest in Istanbul's Taksim square AP In pictures: Turkey coup attempt People demonstrate in front of the Republic Monument at the Taksim Square in Istanbul Murad Sezer/Reuters In pictures: Turkey coup attempt Turkish soldiers detain police officers during a security shutdown of the Bosphorus Bridge Reuters In pictures: Turkey coup attempt Turkish Army armoured personnel carriers in the main streets of Istanbul Getty In pictures: Turkey coup attempt Chaos reigned in Istanbul as tanks drove through the streets EPA/TOLGA BOZOGLU In pictures: Turkey coup attempt Turkish President Tayyip Erdogan speaks to media in the resort town of Marmaris Reuters In pictures: Turkey coup attempt Supporters of President Erdogan celebrate in Ankara following the suppression of the attempted coup Reuters

Underscoring the complicated relationship between the US and Turkey, the White House readout of Mr Trump's call also noted the pressing issues on which they have tried to work together, such as fighting Isis and quelling Syria's civil war.

The White House previously sidestepped questions on the referendum, with spokesman Sean Spicer saying the US wanted to let an international commission review the results.

"They have a right to have elections and their people participated in that," Mr Spicer said. "Before we start getting into their government system, let this commission get through its work."

Irregularities could have changed the outcome of the referendum, Alev Korun, an Austrian member of the Council of Europe observer mission, told ORF radio.

"There is a suspicion that up to 2.5 million votes could have been manipulated," the Austrian member of parliament said.