The Russian ruble on Monday plummeted to a four-month low after stabilising to a degree in spring, hurt by falling oil prices amid a global oversupply of crude.
The ruble hit the psychological threshold of 66 rubles per euro in afternoon trading in Moscow, a four-month low.
The Russian currency also edged closer to 60 rubles per dollar, weakening to 59.8 against the dollar.
The "short-term outlook for the ruble looks cloudy," said Dmitry Polevoi, chief Russia economist at ING.
"The ruble may easily test 60 against the dollar today or in the coming days, and the negative pressure may" continue.
President Vladimir Putin's spokesman Dmitry Peskov said the authorities were watching oil prices closely but stressed it was too early to draw any long-term conclusions.
The Ukrainian crisis has led to unprecedented Western sanctions on the Russian economy and last year's collapse of oil prices plunged Russia into a monetary crisis at the end of 2014 that has now become a deep recession.
The new weakening of the ruble is expected to further erode consumption. It comes after the embattled currency bounced back somewhat in recent months.
Russia's economy contracted by 1.9 percent in the first quarter of the year, the first contraction since 2009.