Comcast said Wednesday it is in "advanced stages of preparing" a "superior" all-cash offer for the parts of Twenty-First Century Fox that Rupert Murdoch's company has agreed to sell to Walt Disney.

Comcast, CNBC's parent company, said in a statement that the offer would top the value of the deal with Disney, but no final decision has been made.

It did say that efforts to finance the bid and submit key regulatory filings are "well advanced."

Shares of Comcast closed 1.9 percent lower Wednesday, while Disney shares fell 1.1 percent. Fox shares gained 1.6 percent.

"Everything I hear indicates that [Comcast CEO] Brian Roberts is extremely focused and aggressive in terms of wanting to own these assets," CNBC's David Faber said Wednesday on "Squawk Box."

However, Fox may find Comcast's bid less attractive than Disney's for tax reasons, Faber said, citing sources. Disney's stock offer would allow Fox to spin off its assets tax free, while a cash offer from Comcast would result in a taxable spin, the sources said.

Disney announced an agreement in mid-December to acquire Fox's movie studios, the Nat Geo and FX networks, regional sports networks and other parts of the company for $52.4 billion in stock. The deal, which included stakes in Sky and Hulu, had a total value of about $66.1 billion.

Such an acquisition would boost Disney's efforts to compete with Netflix in streaming.

"In a nutshell, if Comcast won these assets from the arms of Disney it would be a 'devastating blow' to [Disney CEO Bob] Iger and Disney's streaming ambitions going forward," Daniel Ives, head of technology research at GBH Insights, said in a note Wednesday.

Citing sources, CNBC reported earlier this month that Comcast was preparing to make a bid for the assets in mid-June. CNBC also reported that week that Comcast was preparing an all-cash bid of $60 billion, topping Disney's deal, if the government approved AT&T's acquisition of Time Warner.

Sources also told CNBC this month that Comcast could take on debt of up to $100 billion in the deal.

The Fox assets under consideration do not include the Fox News Channel, Fox Business Network, Fox Broadcasting Company and certain other components.

Read the full press release here.

— CNBC's David Faber, Liz Moyer and Michelle Castillo contributed to this report.

Disclosure: Comcast is the owner of NBCUniversal, parent company of CNBC and CNBC.com.