The top US diplomat to Turkey on Monday called for the protection of fundamental freedoms, no matter whether the NATO ally is ruled by a presidential or parliamentary system.

Ambassador John Bass said it was up to the Turkish people to decide which system they preferred, amid a debate over how the country should be governed ahead of a key parliamentary vote in June.

Opposition parties have challenged President Recep Tayyip Erdogan's plans to switch to a presidential system, claiming he could end up with too much power and usher in authoritarian rule. Erdogan has repeatedly denied the claims.

"We partner with some strong presidential democracies and we partner with some strong parliamentary democracies. From our perspective the system is not the essential piece," Bass told journalists in Ankara.

"The protection of fundamental freedoms, the extent to which that system provides checks and balances that prevent any single individual or any single branch of government from exercising disproportionate influence and ability to rule without consent: those are the kinds of things we look at closely," he said.

Under the current system, the president plays a largely ceremonial role. But Erdogan, who was prime minister for 11 years, has stretched the powers of the presidency since winning it last year, in the first ever direct vote.

The co-founder of the ruling Islamic-rooted Justice and Development Party (AKP), Erdogan wants to cement his role as Turkey's number one with a US-style executive presidency.

The AKP, which currently has 312 seats, is aiming for a significant boost in the June 7 vote. It wants to secure more than two-thirds of the 550-seat parliament in order to change the constitution and switch to a presidential system.

- 'Turkish democracy matters' -

Washington is watching the election campaign with interest, Bass said.

"We do that in part because the quality of Turkish democracy matters very much to us and to our allies," he added.

Turkey has long been accused of violating press freedoms and other rights. The country under Erdogan was branded "not free" by Freedom House, a US-based watchdog.

Bass criticised the government's accreditation ban on some opposition media outlets in the run-up to the key election, saying that it was "very problematic" that voters had to work very hard to get more than one point of view.

"And to the extent that certain viewpoints in journalism or politics are excluded from the legitimate debates and discussions about society, that's a matter of concern," Bass warned.

Asked about Turkey's request for the extradition of US-based Muslim cleric Fethullah Gulen, Erdogan's arch foe, the ambassador only said the United States was studying the request "rigorously, fairly and impartially".

The cleric, 74, was once a close ally of Erdogan. But the authorities blamed Gulen for corruption allegations that rocked Erdogan's government in December 2013 while he was prime minister.

Gulen, who left for the United States in 1999 to escape charges of anti-secular activities by the government at the time, has denied being behind the graft allegations against Erdogan.