ZURICH, Sept 4 (Reuters) - Roche said on Friday it had won breakthrough therapy designation from the U.S. Food and Drug Administration for an experimental haemophilia medicine, aiming for a piece of the $11 billion haemophilia drug market.

The Swiss drugmaker said its U.S.-based Genentech unit's ACE910 secured the fast-track designation as the company prepares separate Phase III trials in 2015 and 2016, the first in patients with haemophilia A with factor VIII inhibitors and the second for patients without inhibitors.

It represents a threat to more traditional treatments from Novo Nordisk and Baxalta, the target of a $30 billion takeover attempt by Shire.

Haemophilia A is a rare genetic disorder that prevents blood clotting. Patients receive lifesaving infusions of clotting factors, but development of inhibitors in many of those being treated interferes with efforts to control their bleeding.

With the market for haemophilia medications expected to grow to $11 billion next year, Roche's ACE910 drug is closely watched because it could change the way the disease is treated.

"FDA has granted breakthrough therapy designation for ACE910, recognising an unmet need for patients with inhibitors and the promise of these early data," Sandra Hornung, Roche's chief medical officer, said in a statement.

Last year, Roche said early data indicated encouraging reduction in bleeding rates in all patients.

In 2012, U.S. regulatory changes created the breakthrough therapy designation, allowing the FDA to expedite development and review of drugs whose preliminary clinical evidence indicates substantial improvement over existing therapies.

(Reporting by John Miller. Editing by Jane Merriman)