Rupert Murdoch's Twenty-First Century Fox has strengthened its offer to protect the independence of Sky's loss-making news channel to try to overcome regulatory concerns about Fox's takeover of the parent company.

Britain's competition regulator has said Fox's $15.7 billion deal to buy the 61 percent of Sky it does not already own should be blocked unless a way is found to reduce the influence Murdoch could wield through the ownership of Sky News.

The objections marked the latest twist in Murdoch's eight-year battle to take control of Europe's leading pay-TV company, forcing Fox to come back with more and more concessions to try to allay concerns about the deal and win regulatory approval.

The takeover is being closely watched in the United States where Murdoch has agreed to sell a string of assets to the Walt Disney company, including Sky, for $52.4 billion.

Last week, Fox pledged to maintain and fund a fully independent Sky-branded news service for five years, and on Tuesday it upped this offer to run for 10 years.

It had already proposed to establish a fully independent board to oversee the 24-hour news network, including the appointment of its head, who will have sole responsibility for editorial strategy and staffing.

It said on Tuesday the Sky News board would now be required to prepare an annual statement confirming it had not come under any influence or attempted influence in the way it creates its editorial output.

"21CF (Fox) is willing to offer the revised undertakings" to address the objections raised by the regulator, Fox's lawyers Allen & Overy said in a letter to the Competition and Markets Authority.