The bidding war for software-maker SumTotal Systems Inc heated up with private-equity firm Vista Equity Partners sweetening its bid by 6 percent, trumping Accel-KKR's offer that SumTotal had agreed to.

Vista, which has a 13 percent stake in SumTotal, raised its offer to $4.75 a share in cash, or about $155 million, from its prior $4.50 a share offer.

Separately, SumTotal said its board determined that Vista's latest offer was superior to the deal it had agreed to with the technology-focused private-equity firm Accel-KKR.

If SumTotal accepts Vista's offer, it would be required to pay a $6.7 million fee to Accel-KKR.

Vista first offered to buy SumTotal in April for $3.25 a share, but was forced to raise its bid to $4.50 a share after SumTotal agreed to be bought by Accel-KKR for $3.80 a share.

SumTotal shares, which have gained about 50 percent of their value in the last one month, were trading up 5 percent at $4.75 in Thursday morning on Nasdaq.

(Reporting by Bijoy Koyitty in Bangalore; Editing by Anne Pallivathuckal and Himani Sarkar)