Image copyright Reuters Image caption The 78-year-old actor has sparked outrage from both sides for his tweet about Barron Trump

Actor Peter Fonda has apologised to President Donald Trump's family for a "vulgar" tweet about his youngest son and the separation of migrant families.

Fonda's tweet, which has since been deleted, said Barron Trump should be ripped from his mother's arms and "put in a cage with paedophiles".

Fonda was one of many celebrities expressing outrage at the White House "zero tolerance" immigration policy.

The Secret Service was alerted to the tweet at Melania Trump's request.

"I tweeted something highly inappropriate and vulgar about the president and his family in response to the devastating images I was seeing on television," the veteran actor and two-time Oscar nominee said in a statement on Wednesday.

"It was wrong and I should not have done it. I immediately regretted it and sincerely apologise to the family for what I said and any hurt my words have caused."

Fonda had tweeted about the president's 12-year-old son in response to the migrant family separations occurring at the US-Mexico border. He then went on to call Mr Trump an expletive before deleting the late-night tweet.

Mrs Trump flagged Fonda to the Secret Service for the threatening comments, according to her spokeswoman Stephanie Grisham.

Ms Grisham called the tweet "sick and irresponsible" in a statement to US media.

Image copyright Reuters Image caption Barron's older brother Donald Trump Jr also condemned Fonda's remarks

Barron's older brother Donald Trump Jr said on Twitter that Fonda was "a bully and a coward" for attacking Barron.

Skip Twitter post by @DonaldJTrumpJr You’re clearly a sick individual and everyone is an internet badass but rather than attack an 11 year old like a bully and a coward why don’t you pick on someone a bit bigger. LMK. https://t.co/8OhiQ0aZmO — Donald Trump Jr. (@DonaldJTrumpJr) June 20, 2018 Report

Fonda, 78, is an Oscar-nominated actor and the son of Hollywood legend Henry Fonda and younger brother of actress Jane Fonda.

He has been a vocal critic of the Trump administration.

On Wednesday Mr Trump signed an executive order reversing the migrant separation policy.

Migrant families will still be detained at the border, but under the new order, they will be detained together barring any child welfare concerns.