For most of the war, Iran had an invaluable cushion: dozens of oil tankers holding millions of barrels of crude sitting safely in waters in Asia. Tehran steadily sold the oil to China, bringing in billions of much-needed cash.

Now, that floating storage is gone. By mid-October, the reserves were fully depleted, according to analysts at Kpler, as a U.S. naval blockade prevented Iran from replacing the crude it normally sells to Chinese refineries.

“There’s no more oil to discharge to China anymore,” said Homayoun Falakshahi, head of crude-oil analysis at Kpler.

The drop in oil revenue is a further blow to Iran’s struggling economy, already facing runaway inflation and widespread shortages. U.S. Central Command, which is responsible for the Middle East, said Saturday that the blockade has denied Tehran at least $25 billion worth of oil revenue, which will, in turn, constrict Iran’s ability to replace destroyed factories and weapons.

U.S. actions have “imposed devastating costs on Iran’s security apparatus,” said Adm. Brad Cooper, head of U.S. Central Command.

Even as the U.S. pressure has choked off Iranian exports, oil from elsewhere in the Middle East is flowing, recently exceeding prewar levels, as U.S. protection helps secure the passage of non-Iranian oil through the Strait of Hormuz.

The floating storage has been part of an elaborate system Iran developed over the years to get oil to its main customer, China, despite U.S. sanctions.

Instead of sending oil directly to Chinese refineries, tankers linked to Iran ferried the crude to waters off the coast of Malaysia and Singapore. They then transferred the oil to other tankers that transported it to Chinese refineries. The subterfuge made it harder for the U.S. to track the flow of oil and punish sanctions violators. The waters off Malaysia and Singapore also became a useful place to store oil.

Oil analysts say Chinese payments for Iranian oil can take around two months to arrive, meaning Tehran may still receive a trickle of funding from past sales in the weeks ahead.

Iran has defied predictions that the blockade would collapse its economy and squeeze concessions on its nuclear program for more than six months. Tehran has stored oil in derelict tanks and trucked it to neighbors as the sea route shuts down.

Still, the overall picture is grim for Iran’s economy. Falakshahi says oil sales represent around 30% to 40% of Iran’s government revenue and are its largest source of foreign exchange. A weakening Iranian currency, which has fallen by a third against the dollar since early July, is contributing to breakneck inflation.

“It looks like the U.S. gamble is that the economy will either push the regime back into negotiations to accept a nuclear deal, or will spark protests internally which the regime will face challenges containing,” Falakshahi said.

Alternative routes to export Iranian oil that don’t go through the Strait of Hormuz, like trucking oil overland, won’t come close to replacing the oil transported by sea, according to analysts.

“Is it working? Yes, it’s working,” said Iman Nasseri, senior vice president of Middle East and Asia at energy-analytics company FGE NexantECA in Dubai, who once worked in the research arm of Iran’s oil ministry. Still, he said the regime has proven resilient in the past. “Let’s wait another year and see if the system can stand.”According to Kpler, by the start of the war in February, Iran had around 135 million barrels located overseas—much of it off the coast of Malaysia and Singapore—likely worth around $10 billion or more after the price of oil surged. When the U.S. first imposed a blockade on Iranian oil flows in April, Iran continued selling oil from this offshore reserve, with stores declining rapidly.

When the U.S. lifted the blockade as part of peace negotiations in mid-June, Iran rushed out more oil, building back its reserves until the U.S. reimposed the blockade in mid-July. Those reserves, which had peaked at nearly 90 million barrels, fell to less than 1 million barrels by Oct. 5, and have since been completely exhausted, according to Kpler.

Emma Li, an analyst at energy tracker Vortexa, also reported a major drop, estimating that there were about 6 million barrels of Iranian oil off Singapore as of Oct. 9. She said Chinese oil refineries are increasingly turning to alternative suppliers, including Brazil and West Africa.

United Against Nuclear Iran, an advocacy group, also calculated that Iran’s oil exports had fallen to close to zero in August and September and that floating reserves in Southeast Asia had likely fallen to 10 million barrels or less by early October, saying the precise total was difficult to know. “They’ve got to be hurting,” said Charlie Brown, a UANI researcher. He said that while he couldn’t say that the reserves had gone to zero, “it seems likely to be well on the way to close to zero.”

Iran’s government insists it can withstand the pressure. At a meeting of the International Monetary Fund in Bangkok last week, Abdolnaser Hemmati, governor of Iran’s central bank, said that despite pressure “the Iranian economy has demonstrated very good resilience, and the claim that Iran’s economy is collapsing is nothing more than an illusion.”