President Barack Obama is not expected to hold sit-down talks with his Turkish counterpart Recep Tayyip Erdogan when he visits Washington later this week for a nuclear security summit, amid deep divisions between the two NATO nations.

Several heads of state are set to attend the summit on Thursday and Friday, but Obama is only scheduled to hold a bilateral meeting with Chinese President Xi Jinping, an official said.

The absence of a meeting with Erdogan in the thick of the fight against the Islamic State group is glaring.

Turkey and the United States are nominally close allies, but tensions have been piqued by Ankara's attacks on Kurdish militants, who are seen by Washington as the best bet for tackling the Islamic State group in Iraq and northern Syria.

Turkey says the groups are linked to the outlawed Kurdistan Workers' Party (PKK), which has fought a long battle for Kurdish independence.

The White House has been increasingly outspoken in recent months about threats to free speech and democracy in Turkey.

Earlier this month the White House called on the Turkish government to respect democratic values, amid allegations of a fresh press crackdown.

"We urge Turkish authorities to ensure their actions uphold the universal democratic values enshrined in Turkey's constitution, including freedom of speech, freedom of assembly and freedom of the press," said spokesman Josh Earnest.

Erdogan's government has been accused of authoritarianism and muzzling critical media as well as lawmakers, academics, lawyers and non-government groups.

That is dramatic change since the early days of the Obama administration, when Turkey was seen as a close partner and potential moderate stabilizing force in the Middle East.

The Turkish government has allowed coalition forces to use bases in southern Turkey to hit Islamic State targets.