"As we have said previously, Microsoft reserves the right to pursue all necessary steps to ensure that Yahoo's shareholders are provided with the opportunity to realize the value inherent in our proposal," Microsoft said in a statement without specifying what its next course of action would be.
In response to Microsoft's statement, a Yahoo spokesman said it stood by its earlier letter on Monday from the Sunnyvale, California company's board that the Microsoft proposal undervalues Yahoo.
Still, analysts say Microsoft will probably raise its bid, originally valued at $31 a share, to at least $35, but could be persuaded to go as high as $40.
Microsoft wants to complete the largest-ever computer technology merger in a bold strategic move aimed at creating a formidable rival to Web search leader Google.
Yahoo said the offer did not properly assess its audience of 500 million users worldwide, investments in its online advertising platform and its ability to generate cash.
Redmond, Washington-based Microsoft now must decide whether to sweeten its offer, launch a proxy fight or, the least likely option, withdraw.
A more hostile alternative could be to propose a tender offer to buy shares directly from Yahoo shareholders.