By Natalia Drozdiak and Eyk Henning 

BRUSSELS--The European Union's antitrust agency signaled Wednesday it would conduct a strict review of Bayer AG's plans to buy rival Mosanto Co. for $62 billion, even though the two companies haven't yet entered formal negotiations to merge.

The comments by EU Antitrust Chief Margrethe Vestager come unusually early as the two companies haven't yet agreed to a deal, let alone formally notified their plans to the European Commission, the bloc's antitrust agency.

"Our final decision [on the Bayer- Monsanto deal] must strictly and impartially apply European merger control rules," said Ms. Vestager in a letter dated June 20th to two members of the European Parliament posted on the Green Party's website.

Bayer declined to comment. Monsanto wasn't immediately reached for comment.

Monsanto twice rejected Bayer's informal $122 a share offer, leading to an impasse in the negotiations earlier this month. Bayer's chief executive Werner Baumann signaled to U.S. investors on a roadshow last week that both companies are back in talks over the potential deal, according to two people present at those meetings.

Investors, however, still await clarity from the EU, where bloc officials have delayed reauthorizing glyphosate, the widely used weedkiller Monsanto markets under the Roundup brand. Glyphosate's authorization for use in the EU runs out at the end of June, and member states have been unable to agree on an extension.

The European Commission, the EU's executive arm, on Tuesday sent a proposal for extending glyphosate's authorization by 12 to 18 months to its 28 commissioners. They now have until Wednesday to object to the extension, but are widely expected to approve it. The extra time would allow the European Chemicals Agency to come up with its own health assessment of glyphosate.

The Green Party lawmakers, Martin Haeusling and Sven Giegold, had called on Ms. Vestager to reject the merger on both "competition and political grounds" in a letter dated from late May.

"It's clear that [Bayer's] takeover of Monsanto, the market leader in seeds, would lead to further market concentration, displacement of other producers and less competition, not only in the EU, but world-wide," the lawmakers wrote.

They said the glyphosate debate shows that European consumers are increasingly skeptical about agro-industrial methods.

Ms. Vestager said she would heed their warnings.

"In our investigation, we will take into account your concerns about the effects of the Bayer-Monsanto merger on prices, the variety of available seed products as well as research and innovation," Ms. Vestager said.

Bayer's bid comes as other rivals in the industry, including Dow Chemical Co., DuPont Co. and Syngenta AG, have also made plans to merge. That could mean that some markets may already be more concentrated by the time Bayer formally notifies any deal with the EU regulators.

But in the letter, Ms. Vestager suggested the EU would consider the context that several mergers in the sector would be taking place at the same time.

"I can assure you that my case teams will examine the three announced mergers in the seeds and agrochemicals sector very carefully,"

The commission has the power to block the deals or demand the parties submit remedies, such as divesting assets, to assuage any competition concerns.

Write to Natalia Drozdiak at natalia.drozdiak@wsj.com and Eyk Henning at eyk.henning@wsj.com

(END) Dow Jones Newswires

June 29, 2016 07:41 ET (11:41 GMT)