The US Coast Guard intercepted an oil tanker off the coast of Venezuela in international waters on Saturday, which the White House said is "a falsely flagged vessel operating as part of the Venezuelan shadow fleet to traffic stolen oil".

China slammed on Monday the US arbitrary seizure of an oil tanker off the coast of Venezuela, saying the move violates international laws.

China always opposes illegal and unilateral sanctions that lack basis in international law and without authorization from the United Nations Security Council, Chinese Foreign Ministry spokesman Lin Jian told a daily news conference.

Beijing also opposes any actions that infringe upon the sovereignty and security of other countries, and rejects all forms of unilateral bullying, he said.

Venezuela has the right to independently develop mutually beneficial cooperation with other countries, Lin said.

China believes that the international community understands and supports Venezuela's position in safeguarding its legitimate rights and interests, he added.

The US seized an oil tanker off the coast of Venezuela on Dec 10, then a second tanker on Saturday, both of which it claimed were transporting sanctioned oil.

"The United States will continue to pursue the illicit movement of sanctioned oil that is used to fund narco-terrorism in the region," US Homeland Security Secretary Kristi Noem posted.

The Venezuelan government condemned the seizures as "a blatant theft and an act of international piracy", and said the act would not go unpunished.

"The Bolivarian Republic of Venezuela categorically denounces and rejects the theft and hijacking of another private vessel transporting Venezuelan oil, as well as the enforced disappearance of its crew, perpetrated by United States military personnel in international waters," a statement from the Venezuelan government said.

US President Donald Trump last week ordered a blockade of US-sanctioned oil tankers that travel to or from Venezuela, and said he was willing to continue seizing more ships.

Critics say Trump is using the blockade to force a change of government in Venezuela, or possibly as a step toward a war with the Latin American country.

Brazilian President Luiz Inacio Lula da Silva said on Saturday that the limits of international law were being tested.

"An armed intervention in Venezuela would be a humanitarian catastrophe for the hemisphere and a dangerous precedent for the world," he said.

Several Latin American countries and regional organizations have denounced the US actions, calling for dialogue and urging the United Nations and the international community to act, Xinhua News Agency reported.

Yvan Gil, Venezuela's foreign minister, claimed the US wants the country's energy resources.

Venezuela has the largest proven oil reserves in the world, and exported an average of more than 770,000 barrels per day last year. The first two seized tankers were each carrying an estimated 1 million barrels of oil.

The US military has built up a significant force in the Caribbean: 15,000 personnel, 11 warships -including the largest US aircraft carrier, the USS Gerald R Ford -plus five destroyers, three amphibious assault ships and two cruisers.

Since early September, the US military has sunk more than 28 vessels — which the US claims have been carrying drugs — in the Caribbean and eastern Pacific Ocean, killing more than 100 people on board.