Iran has targeted US radar and air defence installations in the Gulf, warning the US of "more decisive and powerful" attacks to come as the conflict threatened to widen with a blockade of Saudi ports by Houthi rebels in Yemen.
Iran's Revolutionary Guards said they had attacked and stopped two "non-compliant oil tankers" attempting to transit the Strait of Hormuz, part of an effort to tighten its grip on the vital waterway while the war reignited.
Following the deaths of at least three US soldiers in the past several days, the United States launched a new round of attacks against Iran which it said were "designed to further degrade Iranian military capabilities used to attack commercial shipping" in the strait.
Hours later, the Iranian army said it had targeted US assets in Kuwait and Bahrain, including air defence systems, radar installations and administrative buildings.
"With this radar sweep, the enemy should prepare for even more decisive and powerful waves of drone and missile attacks," the Guards said in a statement carried by state news agency IRNA.
US President Donald Trump warned that Iran would pay "many times over" for every American soldier killed, while Iranian President Masoud Pezeshkian said the conflict had become "a full-scale war".
Nearly 100 US service members have been wounded since 7 July, the Pentagon said, most with minor concussions.
Red Sea threat
Tehran's Yemeni allies, the Houthi rebels, meanwhile threatened a major broadening of the war, announcing they would blockade Saudi ports.
Such a move by the Houthis would put at risk Riyadh's ability to bypass the flashpoint Strait of Hormuz for some of its oil exports and have major repercussions on the global economy.
Saudi Arabia and the Houthis traded fire last week for the first time in years, threatening a 2022 truce, but the rebels have largely remained on the sidelines since Israel and the United States attacked Iran in late February.
Saudi Arabia strongly condemned the threat from what it called the "terrorist Houthi militia" and said it "continues to support the brotherly Yemeni people and their legitimate government".
The Middle East war resumed after an April ceasefire and a June framework deal collapsed.
Iran reimposed its blockade of the Strait of Hormuz, and the US hit back by again blockading Iran's ports.
Oil prices surged briefly to a one-month high before easing slightly early today as the sides battle to control the strait, through which a fifth of world oil travelled in peacetime.
Although far below its pre-war totals, Saudi Arabia has been able to continue exporting millions of barrels of oil a day via its Red Sea port of Yanbu.
Oil markets have for months feared that the Houthis would return to attacking shipping in the Red Sea and Gulf of Aden - as they did during the Gaza war - potentially cutting off yet more crude from global markets.
The Houthis, who have been at war with Saudi Arabia for over a decade, have not said how they will enforce their blockade.
Diplomatic exchanges
In the Strait of Hormuz, meanwhile, the Revolutionary Guards said two tankers attempting to transit the strait "were hit by explosions, resulting in massive fires that brought them to a standstill".
The UK Maritime Trade Operations agency (UKMTO) earlier reported that an "unknown projectile" struck a tanker off the coast of Oman.
UKMTO later reported another tanker off the coast of Oman had been struck by a projectile.
Despite the latest fighting, Iranian foreign ministry spokesman Esmaeil Baqaei said diplomatic exchanges with Washington via mediators were ongoing.
Iran's Interior Minister Eskandar Momeni is in Islamabad for a two-day visit during which he will meet with Pakistan Prime Minister Shehbaz Sharif and powerful army chief Asim Munir, according to Pakistan's interior ministry.
Lebanese President Joseph Aoun is set to meet with Mr Trump in Washington as pressure builds for Beirut to disarm Hezbollah, Israel's key condition before withdrawing from south Lebanon.