Hilary Swank -- I REGRET Attending Party for Alleged Murdering Warlord

Hilary Swank says she probably should have said "no" ... when she was invited to a birthday party for the President of Chechnya last week ... saying she had no idea he's been accused of torturing and killing a bunch of people.

Swank -- along with Jean Claude Van Damme and others -- attended a concert in Chechnya for president Ramzan Kadyrov on Oct. 5 ... and immediately came under fire from human rights groups.

Now, Swank has issued a statement ... saying, "I deeply regret attending this event. If I had a full understanding of what this event was apparently intended to be, I would never have gone."

Swank says at the time she accepted the invitation to the concert ... she had no idea the event was intended to honor Kadyrov.

FYI -- Kadyrov has been accused of torturing and murdering a BUNCH of his political rivals, though Kadyrov says it's all BS ... made up by his enemies to discredit him.

UPDATE: Swank gave a speech at the event ... and even said, HAPPY BIRTHDAY" to the President. When someone on stage asked how Hilary knew about Kadyrov's birthday, she replied, "I read ... I do my research."