* Increase not enough to reach annual budget target
* Net debt/GDP ratio falls slightly to 35.2 pct
BRASILIA, Nov 30 (Reuters) - Brazil's consolidated primary budget surplus rose sharply in October to 12.398 billion reais thanks to a rise in tax revenues due to seasonal factors, the central bank said on Friday.
The increase in the closely watched primary surplus, that measures the excess of revenue over expenditures before interest payments are taken into account, is not nearly enough for the government to reach its annual target of 139.8 billion reais this year.
The government of President Dilma Rousseff has already acknowledged it will have to resort to alternative accounting methods to reach its target.
A sharp slowdown in tax revenues and a string of tax breaks provided by the government to lift a sluggish economy have dragged down primary savings. The primary budget surplus is a gauge closely watched by investors because it measures a country's ability to service its debt.
Brazil's government tends to collect more taxes in October as companies start paying income taxes for the year.
The forecasts of 7 analysts polled by Reuters for the primary budget surplus ranged from a surplus of 10 billion reais to 17 billion reais. In September, the country posted a primary surplus of 1.591 billion reais.
The public debt-to-GDP ratio fell slightly to 35.2 percent in October from 35.3 percent in September.