Updated from 8:50 a.m. EST

Blackstone Group, pressured by a rival offer, increased its purchase price for Equity Office Properties ( EOP) by 11% to $54 a share.

The new all-cash offer is valued at $38.3 billion, including debt. Equity Office's board unanimously approved the amendment to the merger deal and continues to back the Blackstone transaction.

Equity Office agreed in November to be purchased by Blackstone for $48.50 a share. Earlier this month, however, Vornado Realty Trust ( VNO) and partners submitted a $52 cash-and-stock proposal for Equity Office.

"We wanted to put an all-cash deal on the table that shareholders could feel good about, and which was sufficiently compelling to keep us on track for a shareholder vote on Feb. 5 and closing on Feb. 8. Our offer is clearly superior on price, timing, form of consideration and certainty," said Gemma Hart, a Blackstone spokeswoman.

In conjunction with the new bid, Equity Office's board of directors approved an increase in the termination fee payable to Blackstone from $200 million to $500 million.

A Vornado spokesman said the company "is continuing to conduct its due diligence and will respond at the appropriate time."

Shares of Equity Office recently were up $2.08, or 4%, to $54.78.

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