TEHRAN- Iran is increasing oil export to Venezuela, documents seen by Reuters on Tuesday showed.

As reported, Iran is increasing supplies of a key crude grade that Venezuela is using to boost its aging refineries' productivity and free domestic oil for exports.

The two U.S.-sanctioned countries have strengthened energy cooperation in recent years, swapping Venezuelan heavy oil and other commodities for Iranian gasoline, condensate, refinery parts and technical assistance.

The exchange has grown since May when state companies form both nations struck a contract to revamp Venezuela's El Palito refinery, after earlier work at the country's largest facility.

Venezuela's state-run oil company PDVSA is set to receive 4 million barrels of Iranian Heavy crude this month, an increase from 1.07 million barrels imported in June and a volume similar to May, when a supply contract with Iranian state firm Naftiran Intertrade Co (NICO) was signed, one of the documents showed.

The cargoes are expected to arrive in Venezuela's Jose port by the end of the month on Iran-flagged supertankers Herby and Serena, according to the document. The vessels' transponders were last recorded passing near Fujairah, in the United Arab Emirates last month, according to Refinitiv Eikon data.

PDVSA and Venezuela's oil ministry did not immediately reply to requests for comment.

PDVSA is refining the Iranian crude at facilities craving suitable crude to increase output of motor fuels. The supply also is allowing the state-run company to free its lightest grades for blending and exporting.

In mid-June, Iranian Oil Minister Javad Oji met with Venezuelan President Nicolas Maduro in Tehran, during which the two sides stressed the expansion of energy ties, especially in the oil industry.

In that meeting, Oji and Maduro discussed and decided on issues such as fuel supply to Venezuela and also the exports of petroleum products, petrochemical products, and petrochemical catalysts to the south American country.

Other topics discussed at the meeting included decisions on the export of technical and engineering services to Venezuela by Iran with the aim of the reconstruction and modernization of the country’s refineries and the development of its oil and gas fields.

Heading a high-ranking politico-economic delegation Maduro arrived in Tehran on June 10 for an official visit.

The presidents of Iran and Venezuela, as holders of more than 460 billion barrels of crude oil reserves, formally held talks during that visit in which a 20-year cooperation document was inked between the two sides.

The strategic document includes cooperation in the fields of politics, culture, economy, oil, petrochemicals, and tourism.

The document was signed in the presence of President Ebrahim Raisi of Iran and his Venezuelan counterpart Nicolas Maduro.

After the signing of the strategic cooperation document, the two presidents held a joint press conference.

Raisi said Iran's foreign policy has always been focused on building relationships with independent countries.

“Venezuela has shown exemplary resistance to enemy threats,” Raisi said.

Back in May, Oji headed a delegation to Latin American countries including Venezuela to pursue expansion of ties.

Earlier on January, the two countries had signed a memorandum of understanding on the sidelines of the 6th Summit of the Gas Exporting Countries Forum (GECF) in Doha, Qatar, to cooperate in a variety of areas including technical services, technology transfer, training services, manpower training and cooperation in the refining sector.

The document was signed by Oji and Venezuelan Foreign Minister Felix Plasencia Gonzalez.