The 3,000-meter (9,800-foot) mountain is one of the world's most active volcanos, according to the AP.
Activity has risen since an earthquake on May 27 hit a region about 30 kilometers (20 miles) to the south.
The mountain's lava dome has swelled in recent weeks, raising fears that it could suddenly collapse and send scalding clouds of fast-moving gas and debris as far as 12 kilometers (eight miles) away, reaching well into populated areas.
Red-hot lava flowed as far as one kilometer (half a mile) from the mountain's crater, while gas clouds called pyroclastic flows streamed as far as 2-1/2 kilometers (1-1/2 miles) down its southwestern slope.
The Russian missile cruiser Varyag, which is currently staying in the waters of the Mediterranean Sea, blocked the US Navy aircraft carrier strike group in the Adriatic Sea