'It's true: I'm back!' Arnold Schwarzenegger tweets after heart surgery

Quoting his famous 'Terminator' catchphrase, Arnold Schwarzenegger has announced he is back on Twitter after undergoing open-heart surgery last week.

He thanked his team of doctors and nurses, and expressed 'gratitude' for his fans.

"I went to sleep expecting to wake up with a small incision and woke up with a big one - but guess what? I woke up, and that's something to be thankful for," the 70-year-old wrote.

"Thank you to the doctors & nurses. And I'm truly filled with gratitude for all of the kind messages."

The 'True Lies' actor and former California governor had the operation after complications from a less-invasive procedure at a Los Angeles hospital.

His spokesperson, Daniel Ketchell, said on Twitter that on Thursday, Schwarzenegger "underwent a planned procedure to replace a pulmonic valve that was originally replaced due to a congenital heart defect in 1997."

Ketchell said the actor was in "good spirits" following the surgery.