UPDATE 1-Magna, Fiat improve bids as Opel battle heats up

* Bidders improve offers for Opel

* German minister says bids still not acceptable

* Top ministers meet in Berlin on Monday

(Updates with Guttenberg in paras 4-5)

By Nicola Leske and Andreas Moeser

FRANKFURT/BERLIN, May 24 (Reuters) - Magna International (MGa.TO: Quote, Profile, Research) and Fiat (FIA.MI: Quote, Profile, Research) have improved their offers for General Motors GM.N unit Opel ahead of a crucial week in which the German government is expected to decide which bid to back.

German Chancellor Angela Merkel will hold a meeting of top minister on Monday to review bids from Magna, Fiat as well as Belgium-listed industrial holding company RHJ International (RHJI.BR: Quote, Profile, Research). No final decision, however, is expected on Monday.

Italian Fiat improved its offer on Saturday after top German officials said that Magna, a Canadian car parts group, submitted a better plan than its rivals. Magna has also improved its bid, a source close to the negotiations told Reuters on Sunday.

But Economy Minister Karl Theodor zu Guttenberg said on Sunday the three bids were still unacceptable because the government would shoulder too much potential financial risk. He told journalists in Berlin the bids had to be improved.