Agrium, the Canadian fertilizer group, signalled a new phase in its $4bn takeover battle for CF Industries by threatening strongarm tactics to get its US target to the negotiating table.

Agrium said it was "actively considering all available options" after Steve Wilson, CF's chairman and chief executive, refused to enter talks despite apparent enthusiasm for the bid from his investors.

While Mr Wilson had phoned Agrium after learning that holders of 62% of CF shares had tendered to the Canadians' cash-and-shares bid, it had only been reaffirm his rejection of the offer, Agrium claimed.

"Contradicting recent public comments that CF is prepared to engage, [Mr Wilson] stated to me that 'there is no reason to meet because nothing has changed'," Mike Wilson, the Agrium chief executive, said.

"Steve Wilson said he called since stockholders wanted him to engage with Agrium. I do not consider returning my phone calls to say that CF refuses to meet to be engagement, and I don't think CF stockholders will either."

'Unambiguous message'

CF directors were "ignoring" the "unambiguous message" their shareholders had sent them by tendering shares to the offer, the Agrium statement said.

It added: "Agrium remains committed to acquiring CF and is actively considering all available options to give the owners of CF the voice in the company they are entitled to."

The statement did not spell out what options Agrium is exploring, although statements of this kind typically encompass litigation.

Agrium has previously ruled out raising its offer, unless through negotiation with the CF board.

The statement failed to offer information on whether further CF stock had been tendered to the Agrium bid, beyond the 62% surrendered by June 22.

Agrium shares closed $0.85 higher at $41.25 in New York, valuing its bid at $81.25 per CF share.

CF shares ended $0.54 lower at $75.65.