Mo Farah faces a testing head-to-head with top American Alan Webb in the Carlsbad 5,000 metres road race on April 6.

The European 5,000m silver-medallist has accepted an invitation to compete at the California venue - even though he is carefully choosing his competitions in Olympic year.

Farah remains adamant nothing must interfere with his preparations for Beijing, which currently are going well after injury problems.

A hamstring problem prevented him defending his European Cross Country title last December - and he is also an absentee from this Sunday's World Cross Country Championships.

But Farah - the leading European in the recent World Indoor 3,000m final - said: "The Olympics remain my goal and I cannot afford to pick up an injury by competing in Edinburgh."

But he recognises the advantages of an early spring outing on the roads and Webb, the USA's top middle distance runner and mile record holder, will provide him with a stern test.

The course is acknowledged as being one of the fastest in the world, with six men's and women's world records achieved there since its inception in 1986.

That suggests Farah could challenge his British best performance for the distance of 13 minutes 29.7 seconds, which he achieved 15 months ago in Stranolar, Ireland.

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