Shipping through the Strait of Hormuz stalled over the weekend, according to maritime intelligence company Windward, after Iran announced it had again closed the world’s most important oil choke point.

The update comes even as industry trackers showed Iranian tankers continued to sail through the strait, a narrow waterway that typically handles around 20% of the world’s oil traffic.

There was a recovery in oil tanker traffic through the strait immediately after the U.S. and Iran signed a 14-point memorandum of understanding (MOU) last week but the latest data shows this has already hit a snag.

An analysis published by Windward found that a total of 12 ships transited the Strait of Hormuz on Sunday, down from more than 21 the previous day. Five of eight inbound vessels were said to be dark, which is when a ship disables its Automatic Identification System (AIS) transponder to hide its location, identity and destination.

“The current traffic profile: dark, sanctioned, Iranian-linked, resembling the late-blockade baseline more than a functioning open strait,” Windward said Sunday in a social media post.

Trade intelligence firm Kpler said last week that at least 20 tankers transited the Strait of Hormuz on Thursday, reflecting the highest level of traffic since June 2. This was still far below prewar levels, when more than 100 ships transited the strait daily, including dozens of tankers.

A separate analysis published Monday from maritime specialists Lloyd’s List also found that commercial traffic continued to move through the Strait of Hormuz over the weekend, defying Iran’s claims that it had closed the waterway once again.

Iran on Saturday said that it had shut the strategically vital strait, citing ceasefire violations after Israel continued deadly strikes in southern Lebanon.

The U.S. military denied those claims, stating the waterway remained open and that “Iran does not control the Strait of Hormuz.”

At least 15 Iran-flagged Suezmaxes and Very Large Crude Carriers (VLCCs) were outbound from the Gulf of Oman with AIS signals active as of Saturday night, according to Lloyd’s List.

The U.S. and Iran held talks in a Swiss mountain resort on Sunday to build on the memorandum of understanding both parties signed on Wednesday.

Both parties were said to have made progress on reaching a final deal within 60 days during the talks, including the agreement to establish a committee and a mechanism to end hostilities in Lebanon.

A senior Pakistani official and an Iranian official, who were involved in the talks in Bürgenstock, have told MS NOW the talks went into the early hours and were “constructive but tense”.

Under the MOU, both sides agreed to reopen the Strait of Hormuz toll-free for at least 60 days and to end all hostilities, including in Lebanon, where fighting has persisted between Israel and the Iran-backed Hezbollah.

Iran’s Foreign Minister Abbas Araghchi said the country had secured waivers for oil and petrochemical exports, the lifting of the blockade on its ports, the release of some frozen assets and the launch of a reconstruction and development plan.

President Donald Trump had threatened further attacks on Iran ahead of the talks in Switzerland. “Iran must immediately stop their highly paid PROXIES in Lebanon from causing trouble. If they don’t, we’ll hit Iran very hard again, just like we did last week, only harder!!!” Trump said in a social media post on Sunday.

Vice President JD Vance, who led the U.S. delegation at the talks, said he was optimistic about the outcome of the Swiss talks despite Iran’s latest threat to shut the strait.

He also downplayed the impact of violence in Lebanon, saying progress had been made toward ending hostilities there. “These things are always a little bit messy,” Vance said.

— CNBC’s Anniek Bao and Spencer Kimball contributed to this report.