The purchase, unthinkable before longtime treaty ally the United States eased sanctions tied to Russia's war in Ukraine, comes as the import-dependent archipelago scrambles to expand its options in a suddenly volatile oil market.

The Sierra Leone-flagged Sara Sky and its cargo from Russia's ESPO pipeline arrived Monday, with documents showing the consignee as Petron Corp, operator of the Philippines' sole oil refinery, said the source, who asked to remain anonymous as they were not authorised to speak to the press.

The Philippines has seen the price of fuel hit historic highs since the US-Israeli war with Iran forced the partial closure of the Strait of Hormuz, with President Ferdinand Marcos saying the country's dwindling stocks could last about 45 more days.

Presidential spokeswoman Claire Castro later confirmed the Russian oil purchase to reporters in a group chat on a messaging app.

An AFP journalist on Thursday saw the Sara Sky at anchor in Limay port just outside Manila, where the Petron refinery is located. It is believed to be the first shipment of Russian oil to the country in five years.

Ramon Ang, CEO of Petron, told AFP last week the company was "in talks" to potentially purchase Russian oil. Ang declined Thursday to confirm the arrival of the shipment.

President Marcos said on Wednesday that the Philippines was casting a wide net in its search for fuel.

"We have not only gone to our... traditional oil suppliers, we have tried to explore other sources that are not affected by the war that is ongoing in the Middle East," he said in a press briefing addressing the state of emergency.

"Nothing is off the table. We are looking at everything, everything that we can do."

- 'All possible partners' -

Asked about the Russian oil, the Philippines' foreign affairs department told AFP on Thursday the country would cooperate with "all possible partners to contribute to regional and global stability, balanced with our duty to protect and secure Philippine national interests".

The United States this month eased some restrictions on sales of Russian crude, allowing countries to purchase oil that was already at sea until April 11.

Ateneo de Manila University economist Ser Pena Reyes said buying the Russian crude -- equivalent to about two days of national demand -- was a pragmatic move that might help stabilise prices in the short term.

Ongoing geopolitical volatility, however, made Moscow less attractive as a long-term partner, he told AFP.

"Russia can be a useful supplementary supplier... but the Philippines will likely benefit more from a balanced approach, maintaining strong ties with traditional partners while gradually investing in renewable energy and regional energy cooperation," Pena Reyes said.

On Thursday, the Philippines' Department of Energy activated a 20 billion-peso ($332 million) emergency fund that energy secretary Sharon Garin called a "proactive step" to securing fuel supplies.

Garin said earlier this week that the archipelago nation also planned to boost the output of its coal-fired power plants to keep electricity costs down as the war wreaks havoc with gas shipments.

With the cost of liquefied natural gas (LNG) soaring, Garin said the country would "temporarily" be forced to lean even more heavily on the fossil fuel, the main contributor of climate-warming carbon dioxide emissions from human sources.