By Jana Shokair, Enas El Ashry and Phil Stewart

DUBAI/WASHINGTON, May 28 (Reuters) - Iran's Revolutionary Guard said on Thursday it had attacked a U.S. air base after the U.S. military launched what a Washington official described as strikes targeting an Iranian drone operation near the Strait of Hormuz, hours after U.S. President Donald Trump denied a report that he was close to reaching an agreement with Tehran.

The escalation highlights the threats to the fragile ceasefire between the United States and Iran that went into effect in April, weakening hopes for an agreement and causing oil prices to rise again.

The U.S. official, who asked not to be identified in order to speak candidly about military operations, told Reuters that the military shot down four Iranian attack drones and bombed a ground control center in the coastal city of Bandar Abbas that was about to launch a fifth drone.

The official said, "These measures were deliberate and purely for defensive purposes, and aimed at maintaining the ceasefire."

The Tasnim news agency quoted the Revolutionary Guard as saying it targeted a US base in retaliation for an early morning US attack near Bandar Abbas airport. The Revolutionary Guard stated it targeted the US airbase from which the attack on the command and control center near Bandar Abbas was launched.

Kuwait, which hosts a large US base, said it was responding to missile and drone attacks without specifying the source of the attacks.

Israel, which is fighting Iranian-backed Hezbollah militants in southern Lebanon, said sirens sounded to warn of hostile aircraft activity in northern Israel.

Oil prices rebounded after reports of escalating hostilities, following a drop of more than 5% on Wednesday. US crude futures rose by more than 3%, while stocks declined and the dollar strengthened.

Trump: No country will control the strait.

The war has claimed thousands of lives and caused a sharp rise in global energy prices since it began on February 28 with US and Israeli strikes on Iran. Trump has repeatedly said that a deal is imminent.

At a cabinet meeting attended by the media on Wednesday, Trump denied a report broadcast by Iranian state television that he had obtained an informal draft of an agreement aimed at restoring commercial shipping through the Strait to pre-war levels within a month, with Iran and Oman jointly managing the traffic.

Trump said no country would have control of the waterway, and appeared to be threatening Oman, a country with which the United States has decades-long military and economic ties.

He added, "No one will control (the strait)... These are international waters and Oman will act like any other country, otherwise we will have to bomb them. They understand that and they will be fine."

Neither the White House nor the Muscat embassy in Washington has yet responded to requests for comment.

No comment has yet been obtained from Iran's permanent mission to the United Nations.

The Iranian television report on the framework agreement stated that the United States would also lift its blockade on Iranian ports and withdraw its forces from areas adjacent to Iran.

Ebrahim Azizi, head of the National Security Committee in the Islamic Consultative Assembly, said that Trump's "speech" will not force Iran to back down from its demands to enrich uranium, exercise control over the Strait, and lift the sanctions imposed on it.

Azizi said in a post on X, "It is clear that Trump, in his quest to find a way out of this strategic predicament, is oscillating between issuing threats and calling for an agreement."

The strait, through which a fifth of the world’s oil and liquefied natural gas shipments passed before the war, the dismantling of Iran’s nuclear capabilities, and the existing sanctions imposed on Tehran are the thorny points of contention in the talks seeking to end the conflict that began three months ago.

The waterway is subject to international law, which guarantees foreign vessels the right to pass through it.

The U.S. Treasury Department added the "Persian Gulf Strait Authority," the Iranian body established to manage traffic through the strait, to its list of sanctioned individuals and entities that pose a threat to U.S. national security.

Iranian state television said the draft agreement also stipulates the withdrawal of US forces from the region surrounding Iran, but noted that the issue of the US troop presence in the region requires further discussion. The White House denied the report, calling it "completely fabricated." Tehran has not commented.

The Iranian television report on the draft agreement made no mention of Iran’s nuclear program, which the United States wants to dismantle.

Iranian sources said that talks on the nuclear issue would take place in a second round of negotiations—a prospect that might not be acceptable to some of Trump's staunchest supporters. Tehran maintains that its nuclear program is for peaceful purposes only.

"The bottom line is that Iran will never have a nuclear weapon," U.S. Secretary of State Marco Rubio told a cabinet meeting.