Negotiations between the Kurdish Democratic Party of Iran (KDPI) and the Kurdistan Democratic Party-Iran -- both banned in the Islamic republic -- "led to the party's reunification", said the statement issued late Sunday.

Formed in 1945, the KDPI is the oldest Iranian Kurdish party, but most of its members are based in northern Iraq's autonomous Kurdistan region.

The party had led an insurgency against the Iranian authorities since the Islamic Revolution in 1979, and it continues to oppose them from exile.

Assassinations have targeted several leaders of the party, which Tehran considers a "terrorist" organisation.

In 2006, internal disputes led to a faction of the KDPI splitting off and forming the Kurdistan Democratic Party-Iran.

Their reunification "is a new stage in the struggle against the regime of the Islamic Republic of Iran and the mentality that denies Iran's ethnic pluralism and the rights of different peoples", the statement said.

In July, Iran's Revolutionary Guards said they had arrested suspected "terrorists" in the country's northwest, saying they belonged to Kurdish separatist groups based in northern Iran.

Tehran has previously accused "counter-revolutionary" groups in northern Iraq of staging attacks on its territory.

In September 2018, Tehran struck the KDPI's headquarters in northern Iraq, near the border with Iran, killing 15 people.