Actress Sofia Vergara's ex-fiance's demands that she give him two frozen embryos they created - referring to them as "our girls" - has highlighted the wider legal and ethical issues surrounding frozen embryos created by in-vitro fertilisation.

Businessman Nick Loeb has sued the Modern Family star for the embryos because he longs to become a parent and does not want the "two lives" they created to "be destroyed or sit in a freezer until the end of time".

He said he "pushed for children" after he and Colombian-American Vergara got engaged in 2012, and said the couple agreed the following year to try in vitro fertilisation and a surrogate to have children.

Mr Loeb wrote that they signed a form stating that any embryos they created could only be brought to term if both of them agreed. He said the form did not specifically say, as California law requires, what would happen to the embryos if they separated.

He is seeking to have the document voided. He and Vergara split last year and she is now dating actor Joe Manganiello.

Mr Loeb said Vergara's lawyer has said she wants to keep the embryos frozen indefinitely.

Courts have wrangled over what to do with frozen fertilised embryos since at least the early 1990s. But with laws that cover contracts between couples varying by state in the US, consensus is still emerging - and judges are reluctant to compel either a man or woman to become a parent against their will, experts say.

Many judges have looked to a 1992 Tennessee case between a woman in a divorcing couple who sought custody of frozen embryos, and a husband who wanted them destroyed.

The state Supreme Court eventually ruled in his favour, reasoning that the embryos had to be treated differently from property that could be divided equitably, or children, whose custody is awarded based on what is best for them.

But the Tennessee court also left an opening, ruling that if embryos were the only means by which the person seeking them could become a parent, that should carry some legal weight.

In 2012, the Pennsylvania Supreme Court employed just such a test, awarding frozen pre-embryos to a woman who claimed she was incapable of having children after her treatment for cancer.

In Vergara's case, her former fiance wrote that he has offered "to pay for all expenses to carry our girls to term and raise them".

"If she did not wish to share custody, I would take on full parenting responsibilities and agree to have her declared an egg donor," he wrote.