In an open letter on Tuesday, Merck called on Versum's shareholders to put pressure on management to consider the German group's $5.9 billion (4.5 billion pounds) cash bid, including debt, which the U.S. maker of chemicals for semiconductors has spurned in favour of an all-share merger with Entegris .

"We have received renewed encouragement from the capital market for our proposal, following our open letter to Versum shareholders," Merck Chief Executive Stefan Oschmann said at a press conference, reaffirming that his bid was clearly superior.

"The capital market reaction is relatively unequivocal. You see it in Versum's share price," he added, when asked to elaborate.

Merck's takeover proposal of $48 per share in cash compares with an offer of around $39.6 worth of Entegris stock for each Versum share under the existing deal, based on Entegris' closing price of $35.38 on Wednesday.

Versum shares closed at $48.70 on Wednesday, above even Merck's bid and suggesting investors hope for a higher offer.

Oschmann declined to comment on any next steps.

Merck on Thursday reported a decline in adjusted fourth quarter core earnings as adverse exchange rate moves offset demand for its lab supplies and older drugs in China and the Middle East, and forecast a similar effect on 2019 earnings.

Quarterly earnings before interest, taxes, depreciation and amortisation (EBITDA), adjusted for special items, slipped 1.3 percent to a worse-than-expected 950 million euros ($1.07 billion), dragged lower by a weak Argentine peso.

Excluding currency effects, revenue at its healthcare division rose 5.5 percent to 1.63 billion euros in the quarter, on strong demand in emerging markets for its established diabetes and blood pressure drugs and bolstered by recently launched multiple sclerosis drug Mavenclad in Europe.

Revenue at its lab supplies unit surged an underlying 8.8 percent, also to 1.63 billion euros, boosted by demand from biotech researchers.

But weak emerging market currencies cancelled out those gains and family-controlled Merck said it expected a similar burden on 2019 earnings.

Adjusted EBITDA would see a "low teens" percentage rise in underlying terms this year, but foreign exchange headwinds would cause a 3-4 percent drag, it added.

Merck shares were little changed

(Reporting by Ludwig Burger; Editing by Emelia Sithole-Matarise and Mark Potter)

By Ludwig Burger