Several parties have started putting in bids for parts or all of Warner Music Group, the world's third largest music company, sources said on Tuesday.

The bidders, who have had a preliminary review of Warner Music's books, include private equity firms, investor groups and strategic bidders like music companies.

Names that have moved ahead in the process include private equity firm KKR through its BMG Music Rights joint venture with Bertelsmann. Others include billionaire Len Blavatnik, who already owns around 2 percent of Warner Music. Vivendi's Universal Music Group and Sony Corp's label group Sony Music Entertainment and its song publishing joint venture Sony/ATV Publishing are also said to have moved ahead in pursuing their interest.

More official bids are expected this week.

The bids for Warner Music come even as investors are watching out for the next move from Citigroup which took control of rival music company EMI Music last month and is expected to soon put it up for sale.

Most industry watchers see the possibility of both companies being up for sale as a competitive situation which could reduce the respective sellers' bargaining power.

Warner Music Chief Executive Edgar Bronfman is also keeping an eye on the EMI process with the view that his company might in fact end up buying assets of EMI. One scenario could see Warner Music sell its Warner/Chappell song publishing unit and raise enough cash to buy all of EMI.

"They're trying to move closer to pole position to get more financial flexibility to be able to compete with some of the private equity guys who are interested in EMI's assets," said Standard & Poor's Tuna Amobi.

Warner Music has more than $2 billion of debt on its balance sheet and has seen its operating income growth slow down significantly in recent quarters as music sales continue to shrink across the industry.

"The amount of additional leverage they would need to make a bid for all of EMI would be challenging," said Amobi.

But another scenario could see Warner Music's assets ending up being sold to whoever ends up buying EMI.

The number of possibilities highlights the overall weakness and upheaval in the music industry.