The United States on Thursday issued new sanctions targeting Venezuela, imposing curbs on three nephews of President Nicolas Maduro's wife, as well as six crude oil tankers and shipping companies linked to them, as Washington ramps up pressure on Caracas.

The action came as the US executes a large-scale military buildup in the southern Caribbean and as US President Donald Trump campaigns for Maduro's ouster. On Wednesday, Trump said the US had seized a sanctioned oil tanker off the coast of Venezuela.

The US Treasury Department, in a statement, said it imposed sanctions on six shipping companies moving Venezuelan oil, as well as six crude oil tankers that it said "have engaged in deceptive and unsafe shipping practices and continue to provide financial resources that fuel Maduro’s corrupt narco-terrorist regime."

Four of the tankers, including the 2002-built H. Constance and the 2003-built Lattafa, are Panama-flagged, with the other two flagged by the Cook Islands and Hong Kong.

The targeted vessels are supertankers that recently loaded crude in Venezuela, according to state oil company PDVSA's internal shipping documents.