By Ivan Castro

MANAGUA (Reuters) - Nicaragua's highest volcano belched an ash plume up to 3 miles (5 km) into the atmosphere on Saturday, prompting the evacuation of hundreds of nearby residents.

There were no immediate reports of injuries or damage, authorities said. But the government issued a yellow alert, indicating emergency plans had been activated.

The plume formed a cloud extending 30-miles (48 km) from the 5,725-foot (1,745-meter) San Cristobal volcano, which is about 95 miles (154 km) north of the capital Managua in the country's volcano-dotted northwest, said the Nicaraguan Institute of Territorial Studies, or Ineter.

Javier Mejia, executive director of Ineter, which monitors seismic and volcanic activity, said he recommended that civil aviation authorities close down the air space near San Cristobal because of the ash plume and cloud, the largest recorded in recent years.

The volcano has long been active, and stirred in mid-2008, when it expelled gas and rumbled with a series of small eruptions. For months it has emitted "abundant gases in a constant manner," Ineter said in its monthly bulletin.

The government expects to evacuate about 3,000 people from around San Cristobal, though numerous families already have evacuated of their own, said Guillermo Gonzalez, who heads Sinapred, a government emergency and disaster relief agency.

"A response plan exists for volcano eruptions and every community has clearly defined places for people to go to once they are evacuated," Gonzalez said.

In a preliminary report, Ineter said "more gas emissions and sporadic explosions" could be expected from San Cristobal.

The volcano is one of the most active along Nicaragua's Pacific coast, according to the institute, and at times averages nearly 100 seismic movements a day.

Government spokeswoman Rosario Murillo earlier said as many as 20,000 people could ultimately be affected.

(Reporting by Ivan Castro; writing by Simon Gardner and Herbert Lash; editing by Will Dunham and Todd Eastham)