LIMA, Peru — An offshore natural gas exploration rig leased to Venezuela’s national oil company sank off the coast of northeastern Venezuela and forced the authorities to evacuate all 95 of its workers, President Hugo Chávez announced early Thursday.

It was unclear if the sinking posed an environmental risk.

In an attempt to calm nerves after the explosion of an offshore drilling rig last month in the Gulf of Mexico, Venezuelan energy officials said that there would be no harm since security mechanisms were quickly put in place to stop any leaking.

The cause of the sinking was unclear.

Mr. Chávez, who made the initial announcement about the sunken rig via his account on Twitter, also said that two Venezuelan Navy patrols were sent to the waters near the rig, which is owned by a subsidiary of Aban Offshore, India’s largest oil rig company.

“You know this platform is semi-submergible,” Mr. Chávez told his followers on Twitter. “At midnight it listed, took on water, ceased operations and they evacuated,” he said. Mr. Chávez has recently begun using Twitter in an attempt to reach out to people in Venezuela and beyond.