German liquid crystal maker Merck KGaA extended the offer period for its planned takeover of AZ Electronic Materials for a sixth time over the weekend as it continues to seek Chinese antitrust approval.

The offer period has been extended until May 2, Merck said in a statement.

It said China'sMinistry of Commerce, one of the country's three antitrust regulators, had "raised certain specific concerns that Merck is working with the Chinese authorities to address."

"Merck is confident that there will be a satisfactory explanation or solution to these concerns," the statement said.

The world's largest maker of liquid crystals used in TVs, tablet and smartphone screens agreed in December to buy AZ for $2.6 billion to expand its range of specialist chemicals for high-tech gadgets.

AZ Electronic generates the bulk of its revenue in Asia. China's new-found clout in regulating global mergers is causing headaches for some companies seeking deals that need the government's approval.

Merck said it had received valid acceptances of the offer representing approximately 67.53 percent of AZ by Friday.

Merck in March lowered the minimum acceptance threshold among AZ shareholders to 75 percent from 95 percent once the deal had been approved in China.

Bank of America-Merrill Lynch advised Merck on the deal, while Rothschild, Goldman Sachs and UBS advised AZ.