President Donald Trump called to congratulate Turkish President Recep Tayyip Erdogan on Monday after a referendum greatly expanding his powers, according to Turkish officials, despite a more circumspect State Department response to Sunday's vote, which international election observers declared unfair.

Erdogan's office said that he and Trump also discussed the situation in Syria, including the April 4 chemical weapons attack on civilians in Idlib province, and that Trump thanked Erdogan for Turkey's support.

"The two leaders agreed that Bashar al-Assad should be held accountable for the actions he has taken," said a statement from Erdogan's office, referring to Syria's president.

The White House confirmed that the two leaders had spoken, but would not describe the call.

As described by a Turkish statement, Trump's comments differed in tone from those of the State Department, which urged Turkey to respect the basic rights of its citizens and noted the election irregularities witnessed by monitors with the Organization for Security and Cooperation in Europe. The United States is a member of the OSCE.

"We look to the government of Turkey to protect the fundamental rights and freedoms of all its citizens," said the department's acting spokesman, Mark Toner, noting the objections of the Turkish opposition and the monitors.

"The United States remains committed to strengthening our bilateral relationship," he added. "The United States continues to support Turkey's democratic development, to which commitment to the rule of law and a diverse and free media remain essential."

White House press secretary Sean Spicer earlier told reporters the administration would not comment on the referendum until the final report of election observers is complete sometime next week.

"Before we start getting into their government system, let this commission get through its work," he said.

The juxtaposition of the differing responses underscored the awkward situation faced by many U.S. and European officials in responding to the disputed results of the referendum, which changed Turkey from a parliamentary democracy to one led by an executive president with strong central powers. It passed by a slim margin, 51.3 percent to 48.7 percent.

OSCE observers said the outcome did not meet democratic standards, noting that virtually all Turkish media failed to cover the opposition, creating an "uneven playing field."

Erdogan lashed out in response at what he called a "Crusader mentality in the West."

"Both the U.S. and EU are in a bind," said Michael Werz, a Turkey analyst with the Center for American Progress. "They can either [disagree with] the OSCE findings, or they can say the truth: It was not a free and fair election."

Turkey remains a key ally in the campaign against the Islamic State and as the host for millions of Syrian refugees.

The U.S. flies bombing missions over Syria from Turkey's Incirlik Air Base, and almost 3 million Syrians are registered as refugees in Turkey, more than in any other country. The EU has sought Turkey's help in stanching the westward flow of refugees.

Since a failed coup last July, Turkey has been under a state of emergency and basic freedoms have been eroded, according to human rights groups. For example, Turkey has imprisoned more journalists than any other country has previously, according to the Committee to Protect Journalists.

But Turkey's strategic importance means the referendum, even if voting irregularities are proved, is unlikely to impact U.S. policy.

"I don't think the administration will be concerned about problems with Turkish democracy," said Gonul Tol, head of the Middle East Institute's Center for Turkish Studies. "From the get-go, they made it clear they will work with U.S. allies on U.S. national security concerns. But aside from that, they won't interfere in a country's domestic matters."

But Aykan Erdemir, a former member of the Turkish parliament who is an analyst with the Foundation for Defense of Democracies, said suspicions that the referendum was rigged - and still only produced a razor-thin victory - create a legitimacy crisis for Erdogan and his allies in the United States and the EU.

"The ones who voted 'no' are the bulk of the pro-Western and pro-secular crowd in Turkey," Erdemir said. "If the West turns a blind eye to their grievances and embraces Erdogan wholeheartedly, the West risks losing Turkey's pro-West half."

"I understand the U.S. and their EU counterparts have to work with Turkey in the fight against ISIS, and the refugee crisis," said Erdemir. "There needs to be a way to remind Turkey that NATO is not just a tactical alliance of military forces. More importantly, it's an alliance of shared values, of democracy."

Turkish voters approved 18 constitutional amendments that eliminated the position of prime minister and expanded Erdogan's power to appoint judges and prosecutors.

But some observers remain sanguine about Turkey's democratic future.

"As long as Erdogan has an absolute majority in parliament, he can do most anything he wants," said James Jeffrey, a former U.S. ambassador to Ankara and Baghdad. "Look at what [the constitutional change] does. You can overturn a presidential veto with an absolute majority. In America, it takes two-thirds. Are we less democratic than Turkey?"

The most significant change is that Erdogan will have greater latitude in appointing judges and prosecutors.

"Those people are crucial in preserving rights," Jeffrey said. "He has a tendency to appoint people beholden to him, and who think like him. And half of Turkey doesn't."

In the long term, however, the changes may undercut the Turkish democratic model - one that has served as a vivid contrast to the failed governance in many countries in the Middle East.

"The Turkish republic, such as it was, was more important to us than access to Incirlik Air Base," said Eric Brown, a Hudson Institute researcher, speaking Monday at the Bipartisan Policy Center. "It meant having a stable republic, bringing together a diversity of people in a multiethnic society, demonstrating it was possible to have healthy politics. That's no longer there. Turkey is programmed to have greater instability going forward, and much greater frailty."