An activist investment firm Friday urged Yahoo to explore a tie-up with online rival AOL, saying such a deal could help the struggling Internet pioneer "unlock" value for shareholders.

Starboard Value LP, which said it had acquired a "significant" ownership interest in the company, said in an open later to Yahoo chief Marissa Mayer that bringing together the two early Internet giants could lead to "up to $1 billion of synergies" and lift the value of Yahoo as it divests a large portion of its stake in Chinese online group Alibaba.

Silicon Valley-based Yahoo acknowledged it received the letter from Starboard and said that it intends to act in best interests of the company.

"We have maintained, and will continue to maintain, an open dialogue with all of our shareholders," Mayer said in a released statement.

"As part of our regular evaluation of Yahoo's strategic initiatives to drive sustainable shareholder value, we will review Starboard's letter carefully and look forward to discussing it with them."

Starboard's move comes amid intense scrutiny over Yahoo, which like AOL is pushing heavily in digital media as part of reorganization efforts.

Starboard said that the value of Yahoo's core business as measured by its share price is virtually nil, when excluding the valuation of its holdings in Alibaba and Yahoo Japan.

Starboard's analysis said Yahoo's Alibaba stake is worth $34 billion and its holdings in Yahoo Japan valued at $7.8 billion.

With Yahoo's $8.6 billion cash stockpile, its enterprise value should be worth roughly $50 billion, but its market capitalization determined by its share price is just $39 billion, according to Starboard, leaving an $11 billion "valuation gap."

The investment firm said the reason for the gap is that Yahoo has poorly managed both its asset sales and investments, by pursuing a strategy of buying up startups "at massive valuations with seemingly little to no regard for profitability and return on capital."

- End acquisition strategy? -

The most prominent investment by Yahoo was its $1.1 billion deal for the blogging platform Tumblr, which aims at reaching a younger audience.

Starboard said Yahoo should end its "aggressive acquisition strategy" which has spent some $1.3 billion, cut costs and find ways to get value from its overseas holdings with minimal tax consequences.

And it said a tie-up with AOL would be logical because the two firms have a lot of overlap in areas such as online media and advertising.

"Based on our analysis, we believe that a combination of Yahoo and AOL could offer synergies of up to $1 billion by significantly reducing the cost overlaps in their display advertising businesses as well as synergies in corporate overhead," the letter said.

"Importantly, we believe the combined entity would be able to more successfully navigate the ongoing industry changes, such as the growth of programmatic advertising and migration to mobile."

Yahoo shares rose 4.4 percent on the news to close at $40.66.

It is not the first time Yahoo has faced pressure from shareholders.

In 2012, a hedge fund pressed for the ouster of then-chief executive Scott Thompson. And prior to that, billionaire Carl Icahn pressed for a shakeup after Yahoo rejected a takeover bid from Microsoft.

Starboard in 2012 urged changes at AOL, saying the company "remains closed-minded to alternative value creation initiatives."

AOL, formerly known as America Online, has been struggling since the collapse of its leadership as an Internet subscription service, and has been seeking to become a more diversified Web firm. It was spun off from Time Warner in 2009, undoing an unsuccessful 2001 merger.

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