* UN health agency sets up global response unit
* Calls for developing better diagnostics to detect virus
* Some 20-30 monitoring sites could be set up in poor countries
* Learning lessons from Ebola outbreak -senior WHO official
* Mosquito carrying Zika virus seen reaching Asia, Africa (Recasts, adds details, quotes)
By Stephanie Nebehay
GENEVA, Feb 2 (Reuters) - The Zika virus linked to a microcephaly outbreak in Latin America could spread to Africa and Asia, and the World Health Organization will set up monitoring sites in the poorest countries with the highest birth rates, it said on Tuesday.
The WHO on Monday declared an international public health emergency due to Zika's link to thousands of recent birth defects in Brazil.
It called for the urgent development of better diagnostic tests to detect the virus in pregnant women and newborn babies. Seen as a relatively rare condition, the virus may lead to babies being born with small heads and often experiencing neurological disorders and learning disabilities.
"Most important, we need to set up surveillance sites in low- and middle- income countries so that we can detect any change in the reporting patterns of microcephaly at an early stage," said Dr. Anthony Costello, WHO director for maternal, child and adolescent health.
A WHO global response unit "using all the lessons we've learned from the Ebola crisis" has been set up, he said. Some 20 to 30 'sentinel sites' for surveillance could be established worldwide, mainly in poor countries lacking robust health systems.
"Clearly we want to get as many centres as we can collecting the kind of data so that we can pick up any change in the pattern of microcephaly cases at an early stage," Costello told a news briefing.
"The most important thing from my perspective is to see if we can get Zika virus diagnostics improved."
But he added: "It may be too early to pick up associations in cases in other regions, because remember if you're affected early in pregnancy it may take several months before it emerges that there is a case of microcephaly."
The fear is that the disease could travel to other areas of the world where populations may not be immune, he said.
"And we know that the mosquitoes that carry Zika virus - if that association is confirmed - are present ... through Africa, parts of southern Europe and many parts of Asia, particularly South Asia..."
The WHO office for Southeast Asia, in a statement, urged countries in the region to "strengthen surveillance and take preventive measures against the Zika virus disease which is strongly suspected to have a causal relation with clusters of microcephaly and other neurological abnormalities".
Costello said the WHO was drafting guidelines for pregnant women worldwide and rallying experts to work on a definition of microcephaly including a standardised measurement of baby heads.
"At the moment we believe the association is guilty until proven innocent," he said, referring to the connection drawn in Brazil between the virus and babies with small heads.
"Mass community engagement" to rid areas of mosquitoes is needed, he said. "If we learned one thing from Ebola, mobilising communities is an absolute critical public health measure."
"So removing stagnant pools of water in urban areas, looking at upturned flower pots, at rubbish, at tyres, at all the kinds of things that you can do to get rid of breeding sites for the mosquitoes."
Rapid development of diagnostic tools are essential to curbing the virus, especially as a vaccine may be years away, said Costello, a paediatrician.
Sanofi has launched a project to develop a vaccine against Zika, the most decisive commitment yet by a major vaccine producer to fight the disease. (Editing by Hugh Lawson)