UPDATE 1-South Africa's Aspen seeks more time to review Sigma

* Aspen wants to continue due diligence

* Investors speculating Aspen may lower its bid (Adds detail)

SYDNEY, July 6 (Reuters) - Australian drug maker Sigma Pharmaceuticals SIP.AX said on Tuesday its suitor, South Africa's Aspen Pharmacare APNJ.J, had not made a formal offer for the firm but wanted to continue due diligence.

On May 21, Aspen bid A$707 million ($590 million) for debt-laden Sigma and was granted an exclusivity period until July 5. The offer was well below Sigma’s debts at end-January of A$785 million.

Since then, Sigma issued a profit warning that sparked talk Aspen might lower its offer or scrap it.

Sigma shares closed at 39.5 cents on Monday, 34 per cent below Aspen’s proposed offer price of A$0.60 a share, reflecting doubts the deal would proceed.

Aspen, Africa’s largest pharmaceutical firm and one of the world’s top 20 manufacturers of generics, has been in Australia since 2001.

The largest player in Australia’s generic drug industry, Sigma is struggling with a battered share price. It booked an annual loss and its chairman, chief executive and chief financial officer all left the company earlier this year. (Reporting by Michael Smith and Victoria Thieberger; Editing by Balazs Koranyi)