ZURICH (Reuters) - Actelion Ltd, Europe's biggest biotech firm, has won approval from the U.S. Food and Drug Administration to bring a key follow-up to its original lung blockbuster to the American market.

The Swiss company said selexipag, approved to delay disease progression and reduce risk of hospitalisation for pulmonary arterial hypertension (PAH), will become available to patients in the United States in early January 2016.

Its shares rose more than 2 percent in early trading.

The Swiss drugmaker has a dominant position in treatments for PAH, which weakens the heart, and has been banking on new drugs for the condition to cut reliance on its original blockbuster Tracleer that lost U.S. patent protection this year.

Analysts have said that selexipag could reach blockbuster status, meaning annual sales of $1 billion or more.

"Uptravi will significantly expand the options to delay disease progression after initiation of therapy with a baseline treatment like Opsumit and well ahead of Veletri for the late disease stage," Chief Executive Jean-Paul Clozel said.

He was referring to Tracleer follow-up Opsumit that tripled sales to 354 million Swiss francs in the first nine months.

Actelion is facing competition from a new drug called Adempas, sold by German rival Bayer and marketing partner Merck & Co..

Actelion is still working with health authorities to obtain regulatory approval outside the United States.

(Reporting by Brenna Hughes Neghaiwi; Editing by Anupama Dwivedi)