TORONTO - Several Teck Resources shareholders called on Wednesday for Glencore to sweeten its takeover bid for the copper-and-zinc miner, saying the offer is still not high enough for them to oppose Teck's own restructuring plan.

Glencore's unsolicited $22.5-billion bid for Teck last week was the latest in a wave of buyout offers across the mining sector, fueled in part by global opposition to new mine construction and rising demand for copper, a metal key to the green energy transition.

Teck rejected the initial offer twice and said its plan to spin off its steel-making coal unit to focus on copper and other industrial metals was the only viable option for the Vancouver-based company.

Glencore on Tuesday modified the bid to include a cash component, although Teck's board responded by describing it as "largely unchanged".

Glencore CEO Gary Nagle is flying to Toronto to meet with some Teck shareholders on Thursday to personally lobby them to support the deal, in which the combined company would be called GlenTeck.

"The cash component is an improvement, but a higher bid would be required for me to consider changing my vote," said Todd Kapala of Addenda Capital Inc, which holds about 710 000 Teck shares.

Glencore declined to comment.

Another Teck investor said Glencore's offer still did not reflect the expected future value of a company that could become one of the world's largest copper producers.

"The premium doesn't seem sufficient for me to get excited, considering where Teck is headed," said Jonathan Case of CI Global Asset Management, which holds nearly five-million Teck shares, or about 1% of the company's free float.

Teck owns several copper deposits that it can develop into mines, a rarity across the mining industry, Case added.

Glencore's Nagle has called for Teck to postpone its April 26 shareholder vote on its spin-off plan.

A London-based investor holding 700 000 Teck shares said delaying the vote would be a hard sell to shareholders based the current Glencore offer price, even with the cash component.

The investor declined to be named due to sensitivities about discussing an acceptable price.

Canada's Keevil family, which has opposed Glencore's bid, controls Teck through its dominant ownership of "A" class of shares, which have more voting power than the numerous "B" class shares held by institutional investors.