US telecoms company Qwest has increased its offer for rival MCI to $9.75bn (£5.11bn), a premium of 30% over a rival offer from market leader Verizon.
Qwest and Verizon are locked in a bidding war for MCI
It is the latest move in a two-month saga, with Qwest constantly increasing its bid and MCI continuing to recommend a rival bid from Verizon.
Earlier this month, MCI rejected the third offer of $8.94bn from Qwest in favour of $7.51bn from Verizon.
Qwest gave MCI's board until Saturday afternoon to accept its offer.
The board of MCI said it would review the offer.
Both Verizon and Qwest have increased their initial offers for MCI
The market will be waiting to see if Verizon raises its bid for Qwest.
MCI emerged from the ruins of bankrupt telecoms operator Worldcom last year.
Amid a spate of US telecoms mergers, Qwest and Verizon are both keen to win control of MCI's business providing telecom services to big corporations.
The company has in the past said its prospects were better served under Verizon, which is the largest US telecoms operator and which has less debt than Qwest, the fourth largest telecoms firm in the US.
The latest proposal from Qwest is worth $30.40 per share, adding $2.50 in cash per share to its previous offer.
The company said its funding for the bid included $7.25bn in financing, and commitments from 13% of MCI's shareholders to raise $800m by buying additional shares in Qwest at $3.48 a share.