Is Ryan Phillippe's Ex-Girlfriend Pregnant with His Child?

03/14/2011 at 02:25 PM EDT

Just as things are heating up again between Ryan Phillippe and Amanda Seyfried comes a potentially serious complication.

Alexis Knapp, an actress who dated Phillippe last summer, is pregnant and, a source says, "Alexis has told friends that the child is Ryan's."

The baby is said to be due in June.

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Reps for Knapp and Phillippe did not have any comment. But the source tells PEOPLE that "Ryan is aware of the situation [and] totally prepared to take responsibility should the child be his."

Phillippe, 36, who has two children with ex-wife Reese Witherspoon, 34, was spotted last week cozying up to on-again girlfriend Seyfried after the premiere for her movie Red Riding Hood., as well as at the opening of his The Lincoln Lawyer on Thursday.

Knapp appears in two upcoming movies, Project X this Thanksgiving and the Miley Cyrus film So Undercover, also in the fall.