Bayer AG has not offered to pay billions of dollars to settle claims in the United States related to allegations the herbicide Roundup causes cancer, mediator Ken Feinberg said, after a report of such a settlement had driven up shares by as much as 11 per cent.

"Bayer has not proposed paying $8 billion to settle all the U.S. Roundup cancer claims. Such a statement is pure fiction," Feinberg said in an email on Friday. "Compensation has not even been discussed in the global mediation discussions."

The shares, which had shed some gains before Feinberg's statement, declined further and were trading only two per cent above Thursday's close when North American stock markets opened.

Bayer, which acquired Roundup and other glyphosate-based weed killers as part of its $63-billion takeover of Monsanto last year, declined to comment on the initial Bloomberg news report and Feinberg's response.

Bayer CEO Werner Baumann last week said the company would consider settling with U.S. plaintiffs only on reasonable terms, and if it "achieves finality of the overall litigation."

He added at the time the group was "constructively engaging" in a court-ordered process with mediator Feinberg on the cases heard in Federal Court. Most of the pending cases, however, have been filed with U.S. state courts.

Bayer shares have lost more than a third — or roughly $34 billion US — of their value since last August, when a California jury, in the first such lawsuit, found Monsanto should have warned of the alleged cancer risks from Roundup.

The German drugs and pesticides company has engaged in negotiations with plaintiffs' lawyers, a person familiar with the matter told Reuters.

Bayer said this week the next U.S. glyphosate lawsuit scheduled to be heard in St. Louis, Mo., would likely be postponed, and the company added on Friday that the St. Louis case slated for September had been postponed.

The German company may benefit from having cases heard in the city where Monsanto was headquartered and where Bayer now manages its global seeds business — but Missouri is also known for juries that often hit companies with huge damages.

Bloomberg said the delays had been pursued by Bayer to allow for undisturbed settlement talks.

More than 18,000 plaintiffs

The initial unfavourable court rulings in the first three glyphosate cases, heard in California, have at times dragged Bayer's market value below what it paid for Monsanto, although the shares are now trading above that level.

The company, which says regulators and extensive research have found glyphosate to be safe, has previously said it was banking on U.S. appeals courts to reverse or tone down three initial court rulings that have so far awarded tens of millions of dollars to each plaintiff.

Bloomberg cited three sources familiar with the discussions as saying that Bayer's lawyers are seeking an accord to resolve all current and future cases. Talks over cases that have yet to be filed were particularly tricky, the report added.

While Bayer has indicated it could pay $6 billion to $8 billion, plaintiffs' lawyers want more than $10 billion to drop their claims, the report said.

"Progress on settlement is a clear positive to potentially remove what has been the key overhang in shares for a year," Bank of America analysts said in note.

An estimate of a $20-billion hit from the litigation has previously been reflected in the share price, while a likely litigation settlement liability was in the mid-single-digit billion-dollar range, they added.

But they kept a neutral rating on the stock, citing uncertainty over Bayer's fortunes in the appeals process — with the first appeals verdict expected by the end of the year — and whether a settlement could be achieved before that.

The number of U.S. plaintiffs blaming Roundup and other glyphosate-based weed killers for their cancer continued to rise by 5,000 to 18,400, Bayer said last week, as a litigation wave showed no sign of letting up.