Ashton Kutcher and Mila Kunis have been engaged for ''a couple of weeks''.

The 'Two and a Half Men' actor - who was previously married to Demi Moore - and the 'Friends With Benefits' actress sparked speculation last week that they are planning to marry after the brunette beauty was pictured with a ring on her engagement finger, but friends of the couple admit Ashton actually popped the question some time ago, though they still haven't started planning their wedding.

The source said: ''It won't necessarily be a short engagement. They haven't begun planning the wedding yet.''

Ashton, 36, is said to have chosen a ''simple'' engagement ring for the 30-year-old beauty - who he has been friends with since they worked on 'That 70s Show' 16 years ago - and the low-key style reflects their union.

The source told People magazine: ''It's a simple band with a round diamond. No baguettes, no insets. It's not ostentatious, like a Kim Kardashian ring. She doesn't need a security team with her to wear it. It's a lot like her and Ashton.''

The couple - who haven't confirmed their engagement and have rarely spoken about their two-year relationship - spend most of their time together at sporting events or family affairs but they did attend a party in New York last month where they appeared casual, despite being very serious about one another.

The source added: ''They weren't overtly lovey-dovey, but they kissed and you could tell he was proud of her, and she was happy he was there.''