Irans Revolutionary Guards said they seized a United Kingdom-flagged oil tanker in the Strait of Hormuz on Friday, as well as a Liberian-flagged ship later that day. The capture of the tankers, the latest in a series of incidents that have increased international tensions with Iran, came less than 48 hours after Iranian forces captured another foreign ship that they accused of smuggling fuel.

The British tanker, the Stena Impero, was en route to Saudi Arabia when it was seized, and its owners have now lost contact with the vessel, according to the BBC . The U.K. Foreign Office is seeking more information on the ship, and officials held an emergency meeting following Irans announcement. The Revolutionary Guard claimed that the Stena Impero violated maritime regulations, but did not provide any proof of its accusation.

Shortly after Iran claimed to have detained the British ship, British Foreign Secretary Jeremy Hunt confirmed that Iran had seized both the Stena Impero and the Liberian-flagged Mesdar oil tanker.

Irans seizure of the ships coincided with authorities in Gibraltar declaring on Friday that they would extend the confinement of an Iranian oil tanker, the Grace 1 , which British Royal Marines seized earlier this month for allegedly violating sanctions by shipping oil to Syria. Iran has condemned the detention of the Grace 1 and had threatened to seize a British ship in retaliation.

Iran also seized a Panamanian-flagged oil tanker on Thursday morning, broadcasting images of military speedboats circling the ship. Later that day, President Donald Trump claimed that a U.S. warship downed an Iranian drone that came too close to the ship in international waters. Iran denied that it lost a drone, and the U.S. has not released further evidence clarifying the incident.

The U.S. and Iran have been in a tense standoff in recent months after a series of escalating incidents, including the U.S. announcing it would deploy more troops to the region, moving an aircraft carrier strike group to the Persian Gulf and blaming Iran for sabotaging oil tankers. The dispute nearly resulted in the U.S. launching airstrikes against Iranian targets in June, after Iran downed a U.S. drone it claimed had violated its airspace.

Relations between the two countries have deteriorated since Trump withdrew the U.S. from the 2015 nuclear deal last year, resulting in Iran becoming increasingly belligerent as it seeks to push back against U.S. sanctions and military pressure. After complying with the terms of deal even after Trumps withdrawal, Iran surpassed the amount of uranium that the agreement allowed it to enrich earlier this month. The seizure of the Stena Impero and the Mesdar creates yet another problem for diplomats to untangle.