Updated from 3:06 p.m.

Southeastern Asset Management

has amassed a 6% stake in health insurer

Aetna

(AET)

, according to a filing Wednesday with the

Securities and Exchange Commission

, and it hinted that it might be involved with an acquisition proposal for the company.

In its filing, Southeastern said it has held "conversations with third parties" about "proposed corporate transactions of a significant nature" It also said it may choose to "be more active in corporate governance and management matters."

Aetna disclosed on March 1 that it had received a $70-a-share cash and stock offer from

WellPoint Health Networks

(WLP)

and

ING America Insurance Holdings

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, a subsidiary of investment bank

ING Barings

. That proposal values the company at $9.9 billion. The company said at the time that its board was reviewing the offer.

Shares of Aetna closed up 2 15/16, or 6%, to 54 1/8.

Asked whether the board had reviewed the offer letter yet, an Aetna spokeswoman, Joyce Oberdorf, said, "I have no further updates."

The language in the filing is opaque enough, like the language in many filings, that only Southeastern can explain its intentions. After all, it could mean that Southeastern is involved in the WellPoint-ING offer or it could mean that Southeastern has spoken to other companies about acquiring Aetna. "I would be cautious about reading into it that there are clearly other interested bidders," said Dave Shove, an analyst at

Prudential Securities

. He rates Aetna an accumulate and his firm has done no recent underwriting for the company.

A call to Southeastern for comment was not immediately returned.

In any case, Southeastern has revealed itself as an activist shareholder. "The board remains under pressure to realize the value of the franchise," Shove said. "The pressure is not lessening."

Prior to Aetna's disclosure of the WellPoint-ING offer, the Hartford-based company's stock had been depressed for numerous reasons. For example, Aetna reported disappointing earnings for its fourth quarter, and on Feb. 25, the company replaced its chief executive, Richard Huber, with William Donaldson, a co-founder of investment bank

Donaldson Lufkin & Jenrette

(DLJ)

.

According to the SEC filing, Southeastern, which is based in Memphis, now owns 8.47 million shares of Aetna, bought mostly between Jan. 6 and March 2 of this year for a total of $444.34 million.