LONDON, Oct 23 (Reuters) -

Iran said on Tuesday it had arrested 15 militants planning attacks on Shi'ite Muslims making an annual pilgrimage to Iraq.

Hundreds of thousands of Iranians travel to the Iraqi city of Kerbala each year for the ritual of Arbaeen, which marks the end of a 40-day mourning period for the grandson of the Prophet Mohammad, Imam Hussein.

Iran's Intelligence Minister Mahmoud Alavi, visiting the Iran-Iraq border area, was quoted by state television as saying that "three terrorist groups that wanted to target Arbaeen mourners were arrested."

Tasnim news agency quoted Alavi as saying the arrests took place in southwestern Khuzestan province in recent days, and 15 people were arrested.

"The detainees confessed that they wanted to carry out suicide attacks to kill the pilgrims," Alavi said.

He gave no indication when the attacks were due to take place, but the culmination of the pilgrimage this year falls at the end of October.

Shi'ites are considered apostates by hardline Sunni Islamist insurgents in Iraq. Armed Sunni groups in Iran have also increased attacks on military and civilian targets in recent months.

Iran stepped up security in border areas after five gunmen killed 25 people at a military parade in the city of Ahvaz, in Khuzestan province, in September.

Islamic State militants and an Iranian ethnic Arab opposition movement claimed responsibility for the attack, but neither claim provided convincing evidence.

At least 10 Iranian security personnel including Revolutionary Guards were kidnapped on the border with Pakistan last week. A Sunni separatist group said it had seized them as revenge for the oppression of Sunni Muslims.