Sigma Pharmaceuticals (ASX:SIP) has denied market rumours it has received a higher 65¢-a-share takeover bid from South Africa's Aspen Pharmacare Holdings.

Speculations, however, continue that the company has received an informal proposal from Aspen that exceeds last month's 55¢-a-share informal and indicative takeover offer.

Stock prices in Sigma climbed as much as 7¢ or 15.5 per cent to 52¢ Thursday. Shares ended 3.5¢ higher at 48.5¢.

Aspen is believed to have lost patience of the extensive due diligence and negotiations with Sigma and intends to force a decision and put the offer to shareholders.

Sigma said yesterday it was continuing talks with Aspen on a possible deal, but denied receiving an unconditional offer of 65¢ a share.

The rumoured bid would value Sigma at $765 million, compared with $648.2 million under the indicative offer.

Analysts have been suspecting for some time that Aspen might lift its bid after its initial offer of 60¢ a share was decreased to 55¢ last month.

Under the potential agreement, Aspen will have ownership of the entire company, including some of the assets slated for sale.