The US State Department and Pentagon on Tuesday ordered the families of American diplomats and military staff to leave posts in southern Turkey because of security fears.

The move comes amid heightened concerns throughout Turkey and was accompanied by an updated travel warning advising US citizens of an increased threat of attacks, the Associated Press reported.

The State Department and Pentagon said dependents of American staffers at the US consulate in Adana and the Incirlik airbase and two other locations must leave.

"Foreign and US tourists have been explicitly targeted by international and indigenous terrorist organisations," a State Department statement said. "US citizens are reminded to review personal security plans and remain vigilant at all times."

In a statement, the US military's European Command said the step "allows for the deliberate, safe return of family members from these areas due to continued security concerns in the region".

Erdogan heads to US

The order comes as Turkey's President Recep Tayyip Erdogan departed for Washington on Tuesday to attend a nuclear security summit on Thursday and Friday.

There has been intense speculation in the Turkish media as to whether Erdogan would meet Obama, amid signs of a poor relationship with the US over Syria and Turkey's domestic policy direction.

Yet Erdogan, speaking to reporters in Istanbul before his departure, said he would have a bilateral meeting with the US president.

"Our colleagues have planned a meeting with Obama, just as we met at the G20," he said. The two leaders last met at a G20 summit in Turkey in November.

"More than 50 leaders are attending the summit upon Obama's invitation, and we are going to talk with a majority of them. We are going to hold a bilateral meeting with Obama," Erdogan added, saying he did not know how long the meeting would last.

READ MORE: Turkey summons US envoy over support for Syrian Kurds

Obama and Erdogan will hold an "informal discussion", the White House said on Tuesday.

"I would expect that over the course of the visit, the president will have an opportunity at some point to have at least an informal discussion with President Erdogan," White House spokesman Josh Earnest told reporters.

Earnest said the lack of a formal meeting should not be interpreted as a snub to Erdogan, noting the Turkish president will have a formal meeting with US Vice President Joe Biden.

NATO member Turkey is part of the US-led coalition fighting the Islamic State of Iraq and the Levant (ISIL, also known as ISIS), but the two allies are sharply divided over Kurdish fighters in northern Syria.

The Kurdish militia has enjoyed US military support, but Turkey sees it as a threat to its national security.