* OSCE mission in Ukraine could rise to 500 civilian monitors
* Detained military observers were from separate team
* But civilian monitors involved in two incidents on April 27
By Fredrik Dahl
VIENNA, April 29 (Reuters) - The OSCE said on Tuesday it was pressing ahead with plans to recruit hundreds of more monitors for a civilian mission in Ukraine, despite the detention of seven military observers from a separate team captured by pro-Moscow rebels.
The 57 member states of the Organization for Security and Cooperation in Europe, including Russia, agreed last month to send civilian monitors to Ukraine in a bid to help defuse the crisis there. Initially numbering 100 monitors and now totalling 154, it could expand to 500.
A vacancy notice for an extra 400 monitors was issued this month and "the process of secondment/recruitment is ongoing", a spokeswoman for the European security body said in an email.
"The build-up is going on," said Roland Bless, a spokesman for Switzerland which holds the rotating chairmanship of the European security body. "Developments on the ground make it clear you need more monitoring capacity."
The mission is separate from another OSCE-linked mission of unarmed military monitors invited by Kiev, eight of whom were seized by pro-Moscow separatist rebels on Friday. One Swedish officer has been released but four Germans, a Czech, a Dane and a Pole are still being held.
The military monitors, unlike the civilians, were sent to Ukraine under OSCE rules that did not require Moscow's consent.
Russia's OSCE envoy said on Monday it had been "extremely irresponsible" to send them to the tense area, but that it would be good if they were released.
The self-declared mayor of a separatist-held town in eastern Ukraine where the men are being held said on Tuesday he would discuss their release with the West only if the European Union dropped sanctions against rebel leaders.
The rebels have described the military officers as prisoners of war and NATO spies.
The separate civilian monitoring team has avoided such high stakes drama but was involved in two security incidents on April 27 in the eastern Donetsk region. In one case, armed people cross-examined one monitoring team, accusing them of espionage and briefly holding them, the OSCE said.
The OSCE spokeswoman said the "security situation is constantly assessed and operations adjusted accordingly". She gave no detail.
The United States says armed rebels - who have captured towns and government buildings across eastern Ukraine - are operating under the direction of Kremlin agents. Russia denies it is involved and says the uprising is a spontaneous response to oppression of Russian speakers by Kiev.
The OSCE, originally set up during the Cold War as a platform for dialogue between East and West, has played a role in monitoring human rights, elections and other issues, particularly in former Yugoslav and former Soviet states.
Diplomats say its overall work in recent years has been hobbled by renewed East-West tension. (Editing by Peter Graff)
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