US sanctions regulators are increasing pressure on trade with Venezuela, as international tensions escalate over the troubled South American country’s attempts to ship oil to and from Iran and Cuba.

Four crude oil tankers and their registered owner companies were last week added to the list of sanctioned entities maintained by the Office of Foreign Assets Control (OFAC), a powerful authority that sits within the US Department of the Treasury.

The four vessels had “continued to lift oil cargoes from Venezuelan ports”, OFAC says, in one case as recently as April this year. That is despite US economic sanctions put in place in 2017 after Venezuelan authorities brutally repressed protests against President Nicolás Maduro’s government.

“The United States reiterates that the exploitation of Venezuela’s oil assets for the benefit of the illegitimate regime of President Nicolás Maduro is unacceptable, and those that facilitate such activity risk losing access to the US financial system,” the authority warns.

Treasury secretary Steven Mnuchin adds that Maduro’s regime “has enlisted the help of maritime companies and their vessels to continue the exploitation of Venezuela’s natural resources for the regime’s profit”.

The decision closely follows a landmark OFAC advisory focusing on sanctions risks associated with maritime trade. The long-awaited guidance, published in mid-May, targets financial institutions, shipping companies, insurers and other firms that may be exposed to illicit activity at sea.

In this case, the four sanctioned vessels are Athens Voyager, Chios I, Seahero and Voyager I, registered under the flags of Panama, Malta, the Bahamas and the Marshall Islands respectively.

Ship tracking information from Marine Traffic shows that as of press time, Athens Voyager remains at sea close to the United Arab Emirates. Chios I and Voyager I are at ports in Italy and Malaysia respectively, while Seahero has not reported its location since May 31.

Venezuela’s oil sector has come under increasing scrutiny from OFAC since the start of 2019, when state-owned fuel company Petroleos de Venezuela (PdVSA) was added to its sanctions list.

The regulator took the decision to “intensify pressure” on President Maduro, and followed accusations that Venezuelan officials had used the company as a mechanism for laundering the proceeds of corruption and bribery.

Since then, several government officials, vessels and financial institutions have also been added to OFAC’s list – generally in response to exports of crude oil to Cuba, which are also subject to US restrictions.

US officials said Cuban and Venezuelan entities were “changing the names of vessels and facilitating the movement of oil” in breach of its restrictions.

However, tensions have escalated in 2020 as Venezuela’s reliance on international oil trading increased.

In January, PdVSA closed down the only two operating refineries left in the country, reportedly due to multiple failures and shortages of crude. That meant Venezuela – historically reliant on its vast crude oil reserves – became dependent on importing refined petroleum from elsewhere.

Its status as a sanctioned country meant its options were limited, but Iran – itself subject to US restrictions – stepped forward.

In May, it emerged that five oil tankers were undertaking the long journey from Iran to Venezuela, via the Suez Canal, the Strait of Gibraltar and the Atlantic Ocean, carrying two types of refined petroleum.

Three of the vessels – Fortune, Faxon and Forest – were already subject to US sanctions. Along with the remaining two, Petunia and Clavel, the five tankers were all registered under the Iranian flag.

Their journey caused outcry in the US, but despite its heavy military presence in the Caribbean no action was taken. Iran’s President Hassan Rouhani warned that “if Americans create problems for our oil tankers in the Caribbean waters or anywhere in the world, we will reciprocally create problems for them”.

Marine Traffic tracking shows the five vessels were eventually escorted to ports at El Palito, Punta Cardon and Puerto La Cruz with the help of the Venezuelan navy, and Maduro described Iran – along with Cuba, Russia and China – as Venezuela’s “true friends”.

In the weeks since, OFAC has also significantly expanded its sanctions against vessels facilitating petroleum trade with Iran.

121 vessels, including tankers, bunkering vessels and container ships, were this week added to its list due to their links to the IRISL Group, an Iranian shipping lines company.

In Venezuela’s case, however, there is an unusual aspect to the US government’s response. On June 6, US Immigration and Customs Enforcement (ICE) announced that a US$5mn reward was being made available for information leading to the arrest of Joselit de la Trinidad Ramirez Camacho.

Ramirez Camacho currently serves as Venezuela’s “superintendent of cryptocurrency”, ICE says, understood to be a reference to the Maduro regime’s use of petro – a digital token designed to facilitate oil trade outside the formal financial system.

Petro was quickly denounced by US officials upon its launch in 2018, with President Donald Trump signing an executive order banning US entities from transacting in any way with the cryptocurrency.

It appeared to be encountering difficulties in early May, with the Venezuelan government announcing no transactions would be possible for several days due to “maintenance”, but was relaunched later in the month from a new so-called ‘genesis block’ – usually associated with freshly launched tokens.

The government subsequently announced that consumers would be able to pay for fuel directly using petro, though there are still uncertainties around how the token connects to the country’s existing central bank-run payment network, Biopago.