Drugmaker Shire said on Tuesday it was seeking to buy Baxalta, a company spun-off by Baxter International last month, for $30 billion to forge the leading global specialist in rare diseases.

The London-listed group went public with its approach after the US firm turned it down. Shire said its unsolicited all-share offer valued each Baxalta share at $45.23, based on August 3 market prices. Shares in Baxalta jumped 12.7 per cent to $37.35 in early US trading.

The Illinois-based firm, which has a staff of around 16,000, develops biotech treatments for rare blood conditions, cancers and immune system disorders. It had proforma revenue of $6 billion in 2014.

Baxalta offers Shire a promising range of new products to complement its growing portfolio of high-priced treatments for rare, or orphan diseases, said Cowen & Co analyst Christopher Hamblett.

"The deal makes a lot of sense because Shire would be getting some highly durable growth assets," he added.

Shire said it would create a unrivalled rare diseases champion with product sales of around $20 billion by 2020 and double-digit percent sales growth.

The move is the latest in a wave of mergers and acquisitions in the healthcare sector since the start of 2014, stretching from large drugmakers buying up smaller rivals to consolidation among makers of generic medicines and tie-ups between insurers.

Shire's offer of 0.1687 Shire American depositary receipts per share, which represents a premium of 36 per cent over Baxalta's stock price on August 3, would give the US firm's shareholders about 37 per cent of the combined group.

Shire chief executive Flemming Ornskov said he had gone public with its plan, which it first proposed privately last month, after Baxalta's board declined to engage in substantive discussions. - Reuters