LONDON - Advisers to a small band of shareholders in Norwegian video conferencing company Tandberg ASA added their voices to calls for suitor Cisco Systems Inc to up its $3 billion bid, ahead of a Monday offer deadline.

Cisco has made its offer for Tandberg -- its first attempt at a European public takeover -- conditional on 90 percent acceptance.

However, the network equipment maker has run into resistance from roughly 30 percent of Tandberg shareholders. Some previous Scandinavian takeover tussles, such as the purchase of Gambro in 2006, have resulted in higher offers being made.

Tandberg shares were little changed at 152.20 Norwegian crowns ($26.70), up 0.79 percent but below the 153.50 crowns offer price. The stock traded as high as 159.20 crowns in October, on hopes of a higher offer.

In an open letter published on the website of Panta Capital, a small London firm advising on merger arbitrage, it and Zurich-based Scott & Associates AG said the offer did not represent a real premium to market value, as measured by peer valuation, historical trading, or operational performance.

A higher price taking into account its growth profile and the substantial scope for sales and cost synergies, is not in conflict with Cisco's respect of the principles of prudence and financial fairness, the firms wrote, responding to an earlier blog post by a Cisco official.

In a telephone interview, Peter Germonpre of Panta Capital told Reuters a fair value would be at least 170 Norwegian crowns. He said the two firms represented European individual investors holding less than 1 percent of Tandberg stock.

A Nov. 3 Reuters poll of 14 analysts found 12 believed the current offer would not win 90 percent approval. Should that happen, Cisco is faced with a choice of extending the deadline for its existing offer, raising its bid, walking away, or accepting a lower stake in a still-listed Tandberg.

Earlier this week investment adviser OppenheimerFunds said it would not agree to sell the 5.78 percent stake it held on behalf of others, while brokerage SEB Enskilda says investors holding another 24 percent are opposed.

On Wednesday Cisco Chief Executive John Chambers said he believed Cisco would close the transaction but no acquisition was a must have. ($1=5.701 Norwegian Crown) (Editing by Simon Jessop)

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