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 will 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.

White House spokesman Josh Earnest underlined that both Obama and Vice President Joe Biden have met with Erdogan and Turkish officials repeatedly in recent months.

But the absence of a new face-to-face meeting with Erdogan this week, 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 stirred 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," Earnest said.

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

Secretary of State John Kerry did meet his counterpart Foreign Minister Mevlut Cavusoglu on Monday, and both stressed the strength of US-Turkish ties.

Kerry expressed sympathy for Turkey's people and leaders after recent bomb attacks and insisted the two capitals are working together closely to combat the Islamic State group.

"Turkey is an important partner with the United States in this effort. It is a NATO ally," he said.

Cavusoglu agreed, saying: "We have the full determination to defeat the terrorist groups in our neighborhood and all over the world."

The signs of tension marks 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.