All eyes are on whether the United States would join Israel's bombardment of Iran's nuclear and missile facilities, as strikes escalated for a seventh day in the two rivals' conflict, with further damage to atomic energy sites deep in Iran.

Senior US officials are "preparing for the possibility of a strike on Iran in the coming days", Bloomberg News reported, citing anonymous sources.

The situation is still evolving and could change, the report said, with some sources pointing to potential plans for a weekend strike.

"Top leaders at a handful of federal agencies have also begun getting ready for an attack, one person said," it added.

Meanwhile, the Wall Street Journal reported that US President Donald Trump told aides on Tuesday that he had approved attack plans but was holding off to see if Iran would give up its nuclear program.

Trump declined to say on Wednesday whether he had made any decision.

"Nobody knows what I'm going to do," he told reporters. "I have ideas as to what to do, but I haven't made a final decision. I like to make the final decision one second before it's due because things change ... The next week is going to be very big."

Iran's Deputy Foreign Minister Kazem Gharibabadi said on Thursday that his country was ready to defend itself in case of escalation.

"If the US wants to actively enter the field in favor of the Zionist regime (Israel), Iran will have to use its tools to teach a lesson to both aggressors," he said. "Our military decision-makers have all the necessary options on the table."

On Wednesday, Iran's Supreme Leader Ali Khamenei rejected Trump's demand for an "unconditional surrender", calling the ultimatum "unacceptable".

"America should know that any military intervention will undoubtedly result in irreparable damage," he said.

Israel said on Thursday that it had struck Iran's Natanz and Isfahan nuclear sites. A military spokesperson initially said it had also hit Bushehr — the site of Iran's only functioning nuclear power plant — but a military official later said "it was a mistake" to have said this.

Earlier, Israel said it had hit another nuclear site near Arak overnight, where Iran was building a heavy-water reactor. Iranian media reported two projectiles hitting an area near the facility, which had been evacuated, and there were no reports of radiation threats.

The extent of the damage inside Iran from the week-old bombing campaign has become far more challenging to assess in recent days.

The country has stopped giving updates on the death toll, and state media have ceased showing widespread images of destruction. The internet has been almost completely shut down.

Hospital attack

Meanwhile, sirens sounded across Israel as the military said dozens of Iranian ballistic missiles hit at least four sites, including a hospital, and wounded nearly 50 people.

Iran said it was targeting Israeli military and intelligence headquarters near the hospital. The Revolutionary Guards said they had used the ultra-heavy Sejjil missile, with a range of 2,000 kilometers, for the first time.

After the hospital was attacked, Israeli Defense Minister Israel Katz said Khamenei "can no longer be allowed to exist".

"Khamenei openly declares that he wants Israel destroyed — he personally gives the order to fire on hospitals," Katz told journalists in Holon near Tel Aviv.

"Such a man can no longer be allowed to exist."

United Nations Secretary-General Antonio Guterres warned on Wednesday against any "additional military interventions" and called for immediate de-escalation leading to a ceasefire.

In Europe, foreign ministers from Britain, France and Germany, together with the European Union's top diplomat, will hold nuclear talks with their Iranian counterpart in Geneva on Friday, officials and diplomats said.

Agencies and Xinhua contributed to this story.

cuihaipei@chinadaily.com.cn