Three tankers are attempting to navigate the Strait of Hormuz by sailing near the Iranian coast, marking the first vessels to try passing through the waterway since the United States declared a blockade.

The New Future, which has no clear links to Iran, and the US-sanctioned Auroura, began moving northeast early Monday from waters off the United Arab Emirates, according to ship-tracking data, The Caspian Post reports, citing Bloomberg.

The medium-range tankers appeared to have taken a route just south of Iran’s Larak Island, a passage that Tehran has recently said vessels attempting an eastbound transit should follow.

The New Future has already navigated the bend in the strait and is now heading south into the Gulf of Oman, while the Auroura remains near Larak and is moving at an average speed.

At the same time, a Vietnam-flagged and owned liquefied petroleum gas carrier is approaching the strait in the opposite direction, suggesting an attempted entry into the Persian Gulf. The NV Sunshine began sailing northward from waters off Sohar in the Gulf of Oman late Sunday and is signaling that it is headed to Sharjah in the United Arab Emirates.

The transits are taking place just hours before the US is expected to implement a blockade of areas around the Strait of Hormuz, following the collapse of talks between Tehran and Washington over the weekend. The restrictions, which apply to all vessels entering or departing Iranian ports or coastal areas, are set to take effect at 10 a.m. New York time on Monday, after what reports describe as Iran’s tightening control over the strategic waterway in recent weeks.

The movements of the ships are being closely monitored as the US and Iran compete for influence over the chokepoint, through which roughly a fifth of global oil trade has historically passed. In recent weeks, vessels have reportedly been targeted or attacked by Iran over links to Western countries or ownership, while the US move aims to challenge Iran’s control of the strait and reduce its energy revenue.

The New Future is reportedly heading toward Sohar, an Omani port, according to tracking data. It is carrying more than 330,000 barrels of gasoil loaded at Hamriyah in the United Arab Emirates in early April. The vessel entered the Gulf shortly before the start of the conflict in late February.

The Marshall Islands-flagged vessel is owned and managed by Hong Kong Chuanglang Shipping, according to maritime database Equasis. Calls to the company’s listed phone number reportedly failed, with an operator saying the line was inactive due to unpaid bills. The company did not respond to emailed requests for comment.

The Panama-flagged Auroura is signaling that it has Indian crew on board, a commonly used practice by vessels transiting Hormuz to indicate affiliations. The ship was sanctioned by the US in December for links to the Iranian oil trade. Bloomberg News could not immediately verify the cargo’s origin. The vessel has remained in the Gulf since December.

Its owner is listed as Aurora Shipowners Ltd., based in Mumbai, which shares the same address as its manager, Golden Gate Ship Management. No contact details were listed for either company.

Vietnam has recently said it is working with Iranian authorities to ensure safe passage of its vessels through Hormuz. The NV Sunshine is owned and managed by Nhat Viet Transportation Corp., according to Equasis. The Ho Chi Minh City-based company did not immediately respond to calls or emailed requests for comment.