The United States struck southern Iran on Thursday, drawing retaliation from Tehran against a US military base, in the most serious clashes since an April ceasefire began.
The fighting threatened to jettison a fragile diplomatic push to forge a peace agreement and open the Strait of Hormuz, which has become a key point of contention in efforts to formally end the war.
The fresh fighting appeared to begin when Iranian forces fired at four ships attempting to cross the Strait, state broadcaster IRIB reported on Thursday.
US forces launched strikes that hit a ground control station in the southern port area of Bandar Abbas, a US official, speaking on condition of anonymity told AFP, prompting swift response by Iran.
"Following this morning's aggression by the invading US military against a location on the outskirts of Bandar Abbas Airport using aerial projectiles, the American air base that served as the source of the attack was targeted at 4:50 am (0120 GMT)," the Guards said, according to Iranian state broadcaster IRIB.
The Guards did not provide details on the location of the base, though Kuwait's military said its air defenses were responding to an "enemy" attack on Thursday.
The clashes threw into question talks aimed at formally ending the war that began on February 28 with US-Israeli strikes on Iran.
A key focus of the proposed deal has also been restoring full traffic to the Strait of Hormuz, which Tehran has effectively closed, leaving global energy markets grappling with curbed supplies of the huge amounts of oil and gas that normally pass through it.
Oil prices bounced higher on Thursday after reports of the strikes, reversing much of Wednesday's fall on the hopes of a possible imminent deal.
Fresh strikes were also reported in Lebanon, another front of the war.
Israel said it hit the southern city of Tyre, after warning it would take action against Iran-backed Hezbollah and declaring all areas south of the Zahrani River, which lies roughly 40 kilometers (25 miles) from the border, as "combat zones".
The United States Treasury also announced sanctions on Wednesday against Iran's Gulf Strait Authority, Tehran's new agency that collects fees for travelling through the Strait of Hormuz.
- Lebanon operations intensify -
Iran and the United States have traded threats for weeks while negotiating through Pakistani mediation.
Neither side appeared ready to compromise on the main sticking points: Hormuz and Iran's nuclear program.
On Wednesday, the Guards' navy said only ships "willing to abide by Iranian order" could pass through Hormuz.
US Secretary of State Marco Rubio said Tuesday that a deal remained within reach, but that the Hormuz would be reopened "one way or the other".
Iran has also insisted any peace accord must apply to Lebanon, where an April 17 ceasefire has done little to halt fighting between Israel and Hezbollah, which drew the country into the war by attacking Israel in early March in retaliation for the death of Iran's supreme leader.
Israeli Prime Minister Benjamin Netanyahu vowed Monday to "crush" Hezbollah, while army chief Lieutenant Colonel Eyal Zamir said Wednesday that Israel was "intensifying our operations" against the group.
After the warning to evacuate large areas in southern Lebanon, many residents fled to Tyre, according to AFP journalists -- the city subject to new Israeli evacuation orders and where the military said it had begun strikes against Hezbollah infrastructure on Thursday morning.
- 'Nothing is certain' -
In signs of a possible return to normality despite conflict around the region, Iranian authorities partially restored access to the global internet this week after a three-month shutdown.
"I do feel better now because I finally can use my favorite applications," said Hana, a 20-year-old student in Tehran who gave only her first name.
"At the same time, I have this concern that war might resume any minute and just cut me off again from my friends."
Amir, a 27-year-old software developer in the Iranian capital, also feared renewed fighting despite talk of a deal.
"I feel like nothing is certain yet," he said.
"The daily question is: Will there be missile strikes tonight?"
