Turkey is targeting the Syrian Kurdish militia YPG in a weeks-long air and ground operation in Syria's Afrin region.
The Turkish military said one of its helicopters crashed at 1300 (1000 GMT) killing two military personnel but did not give a reason for the incident, only saying that an investigation had been launched.
"We don't have exact evidence or documents to determine that it went down with any outside interference", Binali Yildirim told reporters in the western province of Mugla. The visits by top USA officials come amid Turkey's Operation Olive Branch that has been targeting the PKK terrorist group's affiliates in Syria, the Democratic Union Party (PYD) and the People's Protection Units (YPG), as well as Daesh in northwestern Syria's Afrin region.
Last week Turkish Foreign Minister Mevlut Cavusoglu said he would tell Tillerson during his visit that the two countries must fix damaged mutual trust.
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But it comes as Syrian and Russian forces step up strikes against rebel-held enclaves, Turkey battles US-backed Kurdish forces for a foothold in the north, and US-led forces bombed pro-regime fighters attacking their local allies in the east. "The past week has been one of the bloodiest periods of the entire conflict, with wave after wave of deadly airstrikes leading to civilian casualties in areas of Eastern Ghouta and Idlib", Zeid said in a statement.
"These things will happen, we are in a war. We might lose a helicopter, but they'll pay the price for this", he said in Istanbul.
Sweden and Kuwait presented the measure that would also demand an immediate end to sieges, including in Eastern Ghouta where a bombing campaign by government forces has killed more than 240 civilians in five days. However, the term is usually used for the terrorists who have been killed in the operations.
"This is a threat to Turkey which Ankara has the natural right to fight under global and domestic law", Yildirim added.