Map of the Middle East showing Kurdish distribution.
The Kurds, a non-Arab ethnic group, number between 25 and 35 million people who are spread across four countries but without a state of their own.
Iraqi Kurdistan, an autonomous region in northern Iraq, is to hold a non-binding independence referendum on September 25 that has stirred concern of separatist aspirations in neighbouring states.
The Kurds inhabit mainly mountainous regions that cover almost half a million square kilometres (200,000 square miles), spanning from southeast Turkey through northern Syria and Iraq to central Iran.
They number around 12 to 15 million in Turkey, (about 20 percent of the overall population), six million in Iran (less than 10 percent), 4.7 million in Iraq (15-20 percent), and more than two million in Syria (15 percent).
The Kurds have preserved their culture, dialects and clan-based social structures. Large expatriate communities exist in Armenia, Azerbaijan, Germany and Lebanon.
Although predominantly Sunni Muslims, some are Christians and their political structures are often non-denominational.
Kurdish ambitions of a unified nation are seen as a threat to the main host countries.
- In Turkey, the outlawed Kurdistan Workers' Party (PKK) has been labelled a terrorist organisation by the European Union and United States. More than 30 years of fighting with Turkish forces has killed more than 40,000 people.
- In Syria, the US-backed Kurdish People's Protection Units (YPG) are one of the most effective forces against the Islamic State (IS) jihadist group. They control more than 10 percent of the country in the north and northeast, and three quarters of the border region with Turkey.
- In Iraq, Kurds are an important US ally, and after having resisted the army of dictator Saddam Hussein for decades, now lead the fight against IS.
They control roughly 40,600 square kilometres (15,600 square miles) of territory, including many of northern Iraq's oilfields and the cities of Arbil and Kirkuk.
- In Iran, where the army crushed a fledgling Kurdish republic in 1946, the Party for Free Life in Kurdistan (PJAK) is pushing for autonomy in three provinces.
Kurdish peshmerga fighters are considered to be experienced warriors and Western countries have provided them with air cover, sophisticated weapons and training to combat IS.
Notable Kurdish victories include the YPG's four-month assault against IS fighters in Kobane on Syria's border with Turkey and peshmerga gains in Iraq.
Turkey has regularly attacked YPG positions in Syria since mid-2015.
The Kurds have never lived under a single, centralised power and are split among a myriad of parties and factions.
While some of these groups straddle borders, others are in conflict with each other because of alliances with the governments where they live.
Iraq's two main Kurdish parties, the Kurdistan Democratic Party (PDK) and Patriotic Union of Kurdistan (PUK), were locked in a 1994-1998 conflict that left 3,000 people dead. They reconciled in 2003.