President Nicolás Maduro of Venezuela was indicted in the United States on Thursday in a decades-long narco-terrorism and international cocaine trafficking conspiracy in which, prosecutors said, he led a violent drug cartel even as he ascended to the top of government.

The indictment of a putative head of state was highly unusual and a major escalation of the Trump administration’s campaign to pressure Mr. Maduro to leave office after his widely disputed re-election in 2018. The State Department also announced a $15 million reward for information leading to the arrest of Mr. Maduro, who has led Venezuela’s economy into shambles and prompted an exodus of millions of people.

Mr. Maduro’s government is “plagued by criminality and corruption,” Attorney General William P. Barr said in announcing the charges at a news briefing along with the head of the Drug Enforcement Administration and the top federal prosecutors in Miami and Manhattan, where an indictment accused Mr. Maduro, 57, of importing hundreds of tons of cocaine into the United States.

The Justice Department aimed to root out “the extensive corruption within the Venezuelan government — a system constructed and controlled to enrich those at the highest levels of the government,” Mr. Barr added.