NEW YORK (CNN/Money) - Defense contractor Northrop Grumman Corp. increased its bid to acquire TRW Inc. to $6.7 billion Monday, a rise of 13 percent from the unsolicited $5.9 billion offer the components maker's board rejected a month ago.

The new offer is valued at $53 a share, compared with an original offer of $47 a share.

Shares of TRW rose to $53.05 in pre-market trading Monday from a close of $51.97 on Friday.

Kent Kresa, chairman and CEO of Los Angeles-based Northrop Grumman, said TRW is more valuable because of improving economic conditions.

TRW (TRW: Research, Estimates) late Sunday advised its shareholders in a statement to take no action and said its board would review the revised offer.

Last month, TRW Chairman Phillip Odeen told investors that Northrop's original bid wasn't indicative of TRW's value. The TRW board also urged shareholders to reject Northrop Grumman's hostile tender offer.

"We strongly encourage TRW shareholders to send a strong message to their board of directors in favor of inviting us to conduct due diligence," Kresa said in a news release Sunday. "If the TRW board continues to deny us access, this offer will not proceed."

Northrop (NOC: Research, Estimates) said earlier this month it was prepared to increase its offer if due diligence showed that a higher value for TRW -- a Cleveland-based diversified manufacturer of parts for automobiles, satellites and other machinery -- was warranted.

In response to the takeover attempt, TRW said last month it plans to spin off its automotive parts business within nine months. It said it was in preliminary talks with others who have expressed interest in buying all or part of the automotive business and its aeronautical systems business.

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The automotive business -- best known for air bags, seat belts and sensors -- accounts for 64 percent of the company's sales and 58 percent of profit.

TRW's space and defense business -- the takeover target -- had sales of $5.2 billion last year. Kresa has said Northrop Grumman would spin off or sell the automotive portion after a merger.

Analysts have said that Northrop sought to take advantage of TRW's low share price after the company's CEO, David Cote, left to join Honeywell International Corp. (HON: Research, Estimates)

Northrop Grumman shares fell 57 cents to $118.31 in Friday trading.