(Adds FX volume on spot market, sales to firms)
By Oludare Mayowa and Chijioke Ohuocha
LAGOS, May 2 (Reuters) - Nigeria's central bank is working to make exchange rates for its currency converge on the official and black markets, its spokesman said, and plans to offer $100 million on the forward market on Tuesday to boost liquidity.
Nigeria runs a system of multiple exchange rates, and the central bank has sold more than $4 billion on the spot and forward markets in its efforts to increase liquidity. In theory, greater liquidity should lead the rates to converge.
The naira was quoted weaker on Tuesday at an investor trading window, at 380.31 per dollar, data from market regulator FMDQ OTC Securities Exchange showed. The official market rate was 305.85 and the black-market rate 390.
On the spot interbank market, the naira traded a total of $51.68 million, volumes last seen in October 2016.
"The bank is committed and is indeed working to achieve convergence in the forex rates between the interbank and the bureau de change segment," said spokesman Isaac Okorafor.
The central bank said all dollar allotments must be backed by demand and that it will settle Tuesday's dollar sale between one week and 45 days after the sale.
It then said it would intervene with an undisclosed amount to clear a backlog of demand for airlines, raw materials and machinery and that it would debit the companies for the naira equivalent of the dollar amount immediately.
The bank offered $100 million last Thursday but ended up selling $85.69 million. Okorafor said in a statement lenders did not take up the whole offer as there was "enough foreign exchange to meet legitimate demand".
Analysts doubt whether the central bank can sustain such sales. The convoluted exchange rate system masks pressure on the naira with the regulator trying to avoid a devaluation.
Okorafor said the bank has the capacity to sustain its intervention.
Nigeria's dollar reserves grew to a 19-month high of $30.8 billion last week, thanks to a recent rise in global oil prices, the country's main source of hard currency. But reserves remain far off the peak of $64 billion reached in August 2008. (Additional reporting by Camillus Eboh in Abuja; Writing by Chijioke Ohuocha; Editing by Mark Heinrich)