US defence giant Northrop Grumman has upped the ante in its hostile takeover bid for TRW, taking its offer directly to TRW shareholders.

The latest tactic preceded a vote on Sunday rejecting Northrop Grumman's offer by the TRW board, which deemed the offer insufficient.

Northrop said it made the aggressive move following unresponsiveness by TRW officials to the $5.9bn (�4.1bn) bid.

"We have not received a substantive response from TRW regarding our February 21 letter to enter into negotiations for a proposed business combination," said Kent Kresa, Northrop chief executive.

"Accordingly, we are moving ahead to make this offer available to TRW shareholders," Mr Kresa said in a written statement.

'Grossly undervalued'

TRW believes the takeover amount Northrop has proposed to be too insubstantial for the Cleveland, Ohio-based firm to consider, saying it "grossly undervalues" the assets of the company.

If Northrop Grumman's latest effort succeeds, it plans to spin off TRW's automotive business. The combined defence and aerospace business resulting from the takeover would have a combined turnover of about $27bn.

"We believe the strategic combination of Northrop Grumman and TRW will provide tremendous value to the shareholders of both companies," Mr Kresa said when announcing the bid last month.

Northrop Grumman is famous for its cutting edge avionics knowledge, producing plans like the bat-winged B-2 stealth bomber.

The firm also gained prominence with its Global Hawk reconnaissance aircraft, an unmanned spy plane that can stay in the air for long distances and many hours.

Most recently the plane was used over Afghanistan to hunt for Osama bin Laden.

State law challenged

Aside from TRW's reluctance to accept the offer, Los Angeles-based Northrop also faces other obstacles written into Ohio state law, which was passed with the intent of keeping companies based in the Mid-western state.

Ohio law stipulates that company shares must have been held 60 days prior to the announcement for an offer to takeover.

Northrop Grumman's Kresa said his firm planned to challenge the recently passed statute.

The law was passed just two days before Northrop Grumman made its bid for TRW.