A broad retreat by investors from emerging markets, high oil prices and concerns over potential fallout from an escalating trade war between the United States and China all contributed to the pressure on the rupiah over recent weeks.

The currency lost nearly 2 percent this week, and hit 15,190 to the dollar on Thursday, its lowest in more than 20 years, though it closed slightly stronger on Friday at 15,175.

Indonesia's reserves stood at $114.8 billion at the end of September, equal to six-and-a-half months of imports, BI said in a statement, still far higher than the 3-month international adequacy standard, but representing a 13 percent decline from $132 billion in January.

The September reserves level "is able to support the external sector resilience and maintain macroeconomic and financial system stability", BI said.

Governor Perry Warjiyo earlier on Friday confirmed that BI has been intervening to support the rupiah, though he stressed that BI was also also undertook smoothening operations when trade became lumpy.

This week, the governor said local interest rates must be raised ahead of the US Federal Reserve to avoid "drastic" outflows, in a sign that BI may lift rates further before a widely expected Fed rate hike in December.

The central bank has increased interest rates five times since mid-May, by a total of 150 basis points, to maintain the attractiveness of Indonesian assets for foreign investors.

Separately on Friday, Finance Minister Sri Mulyani Indrawati said authorities were assessing global developments that triggered a rally in the dollar to determine whether they need to take further steps to halt outflows which have pummelled the rupiah.

The government has raised import tariffs, widened biodiesel use and delayed billions of dollars of infrastructure projects to cut import bills and reduce dollar demand onshore.

Indrawati said the government will support businesses to export more and may provide incentives.