(Reuters) - The world's largest agrichemicals company, Syngenta AG , has rejected a $42 billion takeover offer by state-owned China National Chemical Corp, Bloomberg reported on Thursday, lifting Syngenta's shares. Syngenta is under pressure to boost shareholder returns after turning down a $47 billion takeover offer from Monsanto Co this year. Its chief executive stepped down two months later. Citing unidentified sources, Bloomberg said the Swiss-based company was still in talks with ChemChina as well as other suitors, and that a deal could be reached within weeks.(http://bloom.bg/1HL1GRF) Syngenta's Swiss-listed shares opened more than 11 percent higher and were trading up 7.2 percent at 370.90 Swiss francs by 0521 ET on Friday on news of the takeover approach. Bloomberg said Syngenta turned down the offer on regulatory concerns, without elaborating. Asked about the report, a ChemChina spokeswoman said the company had nothing to announce. Syngenta declined to comment. ChemChina has a 5 percent share of the global crop chemicals through its ownership of Israeli generic pesticides maker Adama. Syngenta's 19 percent market share would catapult it to the industry leader position. Acquiring Syngenta would help ChemChina further its international expansion ambitions and help to enhance the technological know-how of China, industry experts said. The company has a history of acquiring interests in Western specialty chemicals businesses. These include Norwegian silicon business Elkem, French feed additives maker Adisseo, Australian plastics maker Qenos and most recently Italian tyremaker Pirelli. "Future demand for pesticides globally will stay strong, particularly for a country like China, which is trying to boost grains production," Duan Yousheng, an analyst with China Pesticides Industry Association, said. "Syngenta has sales channels in over 120 countries and it is a world leader in research, and pesticide sales volume. Only (a)state-owned firm is able to make such offer," he added. ChemChina's initial offer valued the agricultural chemicals group at 449 Swiss francs per share, or 41.7 billion Swiss francs ($41.72 billion), according to Bloomberg. Monsanto had proposed the same price in talks with Syngenta's management earlier this year but wanted to pay 55 percent of that in shares of the combined group. Monsanto in August abandoned a revised cash-and-stock deal proposal that was worth 470 Swiss francs per share. The value of the bid declined to 433 francs during August as Monsanto's stock price fell amid a slump in global commodity demand that is putting pressure on companies for more economies of scale. Before Friday, Syngenta shares were down more than 8 percent since Monsanto walked away in late August. Syngenta spurned Monsanto's approaches as "significantly undervaluing the company". Some shareholders have chided the Swiss group's management for its defensive stance and questioned the company's ability to improve its financial fortunes as demand for agricultural commodities remains weak. To appease shareholders, Syngenta announced plans in September to buy back more than $2 billion of stock, funding the measure by selling its vegetable seeds business. ($1 = 1.0035 Swiss francs) (Reporting by Ismail Shakil in BENGALURU; Colin Packham in SYDNEY and Niu Shuping in BEIJING, Ludwig Burger in Frankfurt; Editing by Miral Fahmy and Jane Merriman)