State-owned China National Chemical Corp. said Monday it was extending its $43 billion agreed takeover for Swiss pesticide and seed giant Syngenta until September.

The company, also known as ChemChina, said the offer for all Syngenta's shares in what would be by far the biggest-ever overseas acquisition by a Chinese firm was now open until at least September 13.

"As previously stated, extensions to the tender offers are expected to occur until all conditions to the offers are satisfied, including obtaining all applicable regulatory approvals," ChemChina said in a statement.

The offer for Syngenta shares, announced in February, had been extended in May and was due to expire on July 18.

"All of the other terms and conditions of the tender offers remain unchanged and ChemChina continues to expect to conclude the transaction by the end of the year," said ChemChina.

Syngenta rebuffed US-rival Monsanto three times last year before accepting the ChemChina's offer.

The proposed merger is not the only one sector, with German chemicals and pharmaceuticals giant Bayer having made in May a $62 offer for Monsanto, although the US company has yet to endorse a deal.