Image copyright Getty Images Image caption Chris Watts avoided death penalty

A Colorado man, Chris Watts, has been sentenced to life in prison for the murder of his pregnant wife and their two daughters.

Shanann Watts, 34, and their daughters, Celeste, three, and Bella, four, were reported missing on 13 August.

Days later the children's bodies were found in oil tanks and their mother buried in a shallow grave.

A judge on Monday described the killings as perhaps "the most inhumane and vicious crime" he had seen.

Watts was handed five life sentences with no possibility of parole, but avoided the death penalty as part of a plea deal on the urging of his victims' family.

He initially expressed shock at their disappearance, even appearing on local television to make pleas for their safe return.

The 33-year-old told police he had informed his wife that he wanted a separation the morning she went missing.

But just three days after they went missing, she and both children were found dead at a remote oil site where Watts worked.

On 6 November Watts plead guilty to nine charges including murder, tampering with deceased bodies and unlawful termination of a pregnancy.

Image copyright Shanann Watts / Facebook Image caption Watts was also given 48 years in his sentence for the death of the couple's unborn child

Police say he admitted he strangling his pregnant wife and smothering their two daughters before disposing of their bodies.

An unsealed arrest warrant revealed Watts had been having an affair with a co-worker.

It has not been confirmed if that woman is a 30-year-old named Nichol Kessinger, who came forward last week in the Denver Post newspaper.

She says the pair only started dating in July, and that Watts told her that he was in the final stages of a divorce.

Ms Kissinger said she only learned of the truth when news of the disappearances and his arrest emerged.

At a sentencing hearing on Monday, Frank Rzucek, Shanann's father, described Watts as a "heartless monster".

"I trusted you to take care of them, not kill them. Then you take them out like trash," he said.

"You have to live with this vision every day of your life, and I hope you see that every day of your life."

Image copyright Getty Images Image caption Shanann's brother Frank Jnr (right) said the family could now begin to grieve

"I didn't want death for you because that's not my right," Sandra Rzucek, Shanann's mother said in her statement.

"Your life is between you and God and I pray that he has mercy for you."

Watts' own parents also spoke at his sentencing.

"As your mother, Chris, I have always loved you and I still do. I hate what has happened," Cindy Watts said, in tears.

A court-appointed lawyer for Watts said: "Although he understands that words are hollow at this point, he is sincerely sorry for all of this."