Turkey is planning to buy four more F-35 Joint Strike Fighters from the US, in addition to the two it had already ordered, the country's defense industry agency said.

"It has been agreed that four more F-35 planes will be procured in addition to the two previously ordered as part of a plan to buy a total of 100 new generation F-35 Joint Strike Fighters," the defense industry executive committee (SSM) said in a statement posted on its website.

U.N. humanitarian chief Valerie Amos is expected to visit Beirut on Thursday to discuss how world powers can support Lebanon which is overwhelmed by the Syrian refugee crisis.

Discussions between Amos and Lebanese officials are expected to focus on the latest measures taken by the Lebanese authorities to control the flow of refugees to Lebanon.

An outlawed Turkish Marxist group on Wednesday said one of its members carried out the deadly suicide bombing at the heart of Istanbul's tourist district that killed the female bomber and a policeman.

The Marxist Revolutionary People's Liberation Party–Front (DHKP-C) said in a statement on its website "our sacrificial fighter... carried out the sacrificial action on the tourist police department in Sultanahmet."

Turkey's constitutional court on Tuesday ruled against lowering the 10-percent election threshold for political parties to enter parliament, media said.

The move could help President Recep Tayyip Erdogan to centralize powers in a powerful presidency, which until he took office in August was largely a ceremonial role in Turkey.

President Recep Tayyip Erdogan said on Tuesday Ankara would "actively" challenge a campaign pressuring Turkey to recognize as genocide the mass killings of Armenians in World War I, on the 100th anniversary of the tragedy this year.

"I believe that both the foreign ministry and the relevant institutions will actively counter those allegations," Erdogan told Turkey's ambassadors in a keynote speech, adding that discussions were already under way to detail an action plan.

A female suicide bomber on Tuesday blew herself up in the most-visited tourist district of Istanbul, killing herself and a Turkish policeman, officials said.

The woman went into the police station in the Sultanahmet district and told the police in English she had lost her wallet before setting off her explosives, governor Vasip Sahin said on Turkish television. 

Turkish President Recep Tayyip Erdogan on Tuesday warned the European Union it should crack down on "Islamophobia" amid rising anti-Muslim protests instead of trying to teach Turkey lessons about democracy.

Erdogan told Turkey's ambassadors posted abroad in a speech in Ankara that they should pursue an assertive foreign policy to represent strong and self-confident "new Turkey" under his rule.

A Dutch reporter based in Kurdish-majority southeastern Turkey on Tuesday said she had been arrested on charges of making propaganda for a "terrorist group."

Frederike Geerdink, who writes for Dutch, Turkish and English-language media, wrote on Twitter that she had been arrested following a search of her house.

The world is losing its ability to prevent conflicts and the lack of "effective leadership" has led to the worst displacement situation since World War II, the U.N. refugee chief warned Tuesday.

Antonio Guterres, the U.N. High Commissioner for Refugees, said in Ankara that the twin crises in Iraq and Syria had created a serious displacement situation and the world showed no effective leadership to address the challenges.

The United States has expressed concern over a decision for Syrians to obtain visas that sharply limit the time they can stay in Lebanon but Social Affairs Minister Rashid Derbas reiterated that no Syrian would be deported from the country under the new rules.

“We are very concerned new visa requirements for Syrians entering Lebanon will create additional challenges for refugees fleeing the Syrian conflict,” State Department spokeswoman Jen Psaki said Monday.