Axa SA, France's biggest insurer, and Australian wealth manager AMP may sweeten their bid for Axa Asia Pacific Holdings to about A$12.4 billion ($15.5 billion) after a first offer was rejected, Citigroup says.

Axa SA, based in Paris, and Sydney-based AMP, whose first offer valued Axa Asia Pacific at A$5.34 a share, may raise the bid to A$6, Citigroup analysts led by Nigel Pittaway said in a note.

Gains yesterday by AMP's stock swelled the value of the offer to A$5.64, Citigroup said.

"The next move seems likely to be a lift in the amount offered," Pittaway wrote in the report. "One wonders for how long Axa Asia Pacific would then be able to hold out."

Axa Asia Pacific soared in Sydney trading on Monday as investors bet on a second bid, and analysts at Credit Suisse Group and Royal Bank of Scotland Group said they also expected a higher offer.

Already Asia's largest takeover offer this year, Axa SA plans to sell its 54 per cent stake in Axa Asia Pacific to AMP and buy back the Asian units for A$7.7 billion.

The current bid consists of 0.6896 AMP shares and A$1.3796 in cash for each Axa Asia Pacific share.

Sarah Hudson, a spokeswoman for AMP in Sydney, said yesterday the offer was "fair value and compelling".

Axa Asia Pacific rose A7c to A$5.77 yesterday in Sydney trading after jumping 33 per cent on Monday.

AMP advanced 3.6 per cent to A$6.34 following a 4.3 per cent gain yesterday. Axa SA added 0.4 per cent yesterday in Paris trading.

AMP could raise the stock component of the offer 4 per cent to 0.7172 of an AMP share and still boost earnings by the third year after the acquisition, Arjan Van Veen, an analyst at Credit Suisse, said in a report.

Axa SA and AMP must win approval from Axa Asia Pacific's independent directors and minority shareholders to carry out the purchase.

It may be "difficult" for those directors to reject a second offer of A$6 a share or more, said John Heagerty, a Sydney analyst at Royal Bank of Scotland. A second offer was likely, he said.

Axa Asia Pacific employs more than 2300 people in Australia and New Zealand.