The Russian ruble will lose its usual support from monthly tax payments after the finance ministry starts buying foreign currencies on the open market to boost liquidity, Russia's deputy finance minister said Thursday.

The ruble usually firms towards the end of every month as export-focused companies convert dollar and euro revenues into rubles to meet local tax duties. In the second half of 2013 however, the finance ministry will deprive the ruble of such support as it plans to start converting oil and gas revenues back to foreign currencies and will channel the money to the reserve fund, a shield against external shocks.

Alexei Moiseev said the mechanism will be similar to the central bank's planned interventions that the regulator used to carry out to smooth out ruble volatility.

"I realize that the ruble moved yesterday on the back of the news. But this [finance ministry plan] is proper from a liquidity point of view," Mr. Moiseev said.

"All we do, we do in order to smooth out volatility of interbank rates. All what happens to the ruble is a side effect and it will also smooth out ruble volatility," Mr. Moiseev added, saying that the ministry's plan should be positive for the Russian bond market.

The ruble weakened Wednesday to its lowest levels versus the dollar since November amid growing fears that the central bank will give in to growing political pressure to ease monetary policy as an economic slowdown deepens.

Mr. Moiseev said that the finance ministry will buy foreign currencies on a regular basis in the amount of a planned transfer to the country's reserve fund divided by the number of working days.

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