In an effort to force Veolia to negotiate, the Suez board has set up a Dutch foundation to prevent the sale of French water assets that its rival would need to divest to win antitrust approval to buy Suez.
Suez also proposed last month to negotiate a takeover if the two companies agreed on the sale of most of Suez's French assets to investment funds Ardian and Global Infrastructure Partners (GIP). Veolia had rejected the proposal.
"The search for a negotiated resolution between the parties is entirely legitimate, but it must respect the principles of transparency and integrity of the market, fairness in transactions and competition, and the free play of offers and higher bids," AMF said in a statement.
AMF is still due to issue its view on Veolia's offer.
Suez said the AMF decision-making process was unacceptable as it didn't respect its rights.
"Suez strictly complies with legal rules in force," the company said in a statement.
Veolia has been locked in a battle for months with its smaller rival Suez, which opposes its moves to try to take over the company in an increasingly hostile saga.
Veolia is Suez's top stakeholder with 29.9% and has launched a 11.2-billion-euro ($13.21 billion) bid for the firm, but under European Union merger rules it cannot exercise control or influence over its target until the deal is cleared by regulators.
Shares in Veolia were up 2.15% at 1353 GMT.
(Reporting by Gwenaelle Barzic, Benjamin Mallet and Matthieu Protard; Editing by Benoit Van Overstraeten, Kirsten Donovan, Andrew Cawthorne)