Washington - The Trump administration began an urgent debate Friday over how to respond to what officials say has grown into a shadow war with Iran, after attacks on oil tankers in the Arabian Gulf that appeared meant to assert Iranian control over one of the world’s most strategic shipping lanes at a time of heightened tension with the United States.

President Donald Trump put Iran on notice that the United States would push back but offered no details and suggested that he was ready to engage with the Iranians, who denied responsibility for the attacks, whenever they are prepared to talk.

But tension remained high, with a senior official confirming that Iran had fired a surface-to-air missile Thursday at a US drone flying over the Gulf of Oman, where the attacks on the tankers occurred. The episode took place early that morning, between the distress calls from the two ships crippled by explosions that day.

Officials at the Pentagon weighed tactical responses to the attacks, like beefing up the security around tankers, or more drastic moves, like deploying as many as 6,000 additional Navy, Air Force and Army personnel to the Gulf.

One of the two tankers hit by the explosions Thursday, the Japanese-owned Kokuka Courageous, was being towed into a port in the UAE for further inspection into how the attack was carried out and with what kind of weapon. The Navy dispatched a bomb squad team to investigate.

The other tanker, the Norwegian-owned Front Altair, remained adrift, on fire and abandoned by its crew after Iranian patrol boats chased off civilian tugs that had come to tow it to port.

Trump, citing a grainy, black-and-white US military video of a small boat filled with sailors at the side of one of the stricken tankers, declared that there was no doubt that Iran was behind the attacks. One of the mines, he said, had “Iran written all over it”.

But others said the footage fell short of proving Iran’s culpability.

Germany’s foreign minister said the video was “not enough” to determine conclusively that Iran had carried out the attacks, a position echoed by Norway’s government, and the European Union cautioned against further escalation. The Japanese owner of the Courageous questioned accounts that the ship had been damaged by a mine or mines, saying it had been struck by a flying object.

Their scepticism reflected a deeper distrust of a US administration that has pulled out of the Iran nuclear deal, spurning its European allies and sowing suspicion that the United States is spoiling for a fight with Iran.

With the flurry of questions about Iran’s motives and the United States’ intelligence, even the president appeared to be treading carefully. While he said the United States would not allow Iran to shut down the Strait of Hormuz, a key transit point for oil shipments, Trump insisted he was not looking for war. He even reopened the door to some kind of engagement with the Iranian leadership.

“I’m ready when they are,” Trump said in a telephone interview Friday with “Fox & Friends,” the Fox News morning program. “Whenever they’re ready, it’s OK. In the meantime, I’m in no rush.”

Trump’s remarks were more cautious than those of Secretary of State Mike Pompeo a day earlier, and they captured a tension in the administration. The president has signaled a desire to reduce US involvement in wars and engage in diplomacy even as he has taken aggressive positions in confronting rivals like Iran. His more hard-line advisers, including Pompeo and the national security adviser, John R. Bolton, are pushing for the United States to tighten the pressure.

US military and intelligence officials expressed a high level of confidence in Tehran’s involvement in the attacks. While the black-and-white video of a group of men on a boat pulling a mine off one of the tankers is the most public evidence, military and intelligence officials said other streams of intelligence showed Iran’s intent to demonstrate control over the flow of oil from the Gulf.

“There’s no one else in the region that really attacks ships in the water - it’s not an easy thing to do,” Vice Adm. Kevin M. Donegan, a former 5th Fleet commander, said in a telephone interview.

US intelligence agencies believe Iran wants to use covert attacks on shipping to drive up the price of oil. That is aimed at hurting Trump, who has tweeted about high oil prices. Iran’s own oil exports are under pressure by US sanctions, and Tehran is hoping to squeeze as much money out of the limited crude it can sell, a goal that would be advanced if oil prices are driven higher by uncertainty about shipments through the Gulf.

Oil prices rose briefly after Thursday’s attacks, although trade tensions with China continue to pressure prices downward. Over time, analysts said, Iran may be trying to push up prices by raising insurance premiums on tankers in making their voyages more treacherous.

While Iran would be at a substantial disadvantage in a full-scale war with the United States, adversaries over the past two decades have shown how the United States struggles with opponents who wage unconventional “shadow wars,” where the definition of victory is not clear and the traditional rules of war do not apply.

This week’s attacks were a step beyond the sabotage of four other tankers in May. They caused greater damage to the vessels and forced the crew of one to abandon ship. The strikes, according to analysts and former officials, appeared meant to demonstrate Iran’s regional strength but still stop short of a US military reprisal.

“They don’t want to do something that would provoke a response that could end their regime,” said Daniel Hoffman, a former CIA officer and Middle East department chief. “They know a war is regime suicide, so they won’t reach a tipping point of an attack that would do so much damage that the US would respond with a large amount of force.”

Bolstering security around tankers passing through the Strait of Hormuz is another option to respond to the attacks, according to US diplomats. An international warning to Iran or its proxy forces that any small boat approaching a tanker could be subject to military action could help deter attacks.

But making such a warning credible is likely to require a broad international coalition. Even if additional forces are sent, the United States alone does not have enough ships to enforce what would amount to a security cordon around all tanker traffic.

On Friday, Jonathan Cohen, acting US ambassador to the United Nations, discussed the situation with Iran with members of the Security Council.

Diplomats said an international anti-piracy mission off the coast of Somalia could be a model for the Gulf and the Gulf or Oman, where the attacks occurred. Winning Chinese support for such an operation would be vital, according to some Us diplomats. In the past, the People’s Liberation Army’s navy has taken part in anti-piracy operations, including off the coast of Somalia.