Israel reportedly lacks the means to take out some of Iran's nuclear sites without US military involvement

President Donald Trump has secretly approved plans for a direct US attack on Iran but has yet to give the order to proceed, The Wall Street Journal reports. Tehran has warned that any military intervention by Washington will be met with "irreparable damage" to the US.

Trump has made direct threats against Iran in recent days, while echoing Israeli claims that the Islamic Republic is developing a nuclear weapon. Iran has repeatedly insisted that its nuclear program is entirely peaceful.

The US president is hoping that the prospect of the US joining Israel's nearly week-long bombing campaign will force Tehran to meet his demands for unconditional surrender, WSJ wrote on Wednesday.

Trump told senior aides late on Tuesday that he has endorsed plans for an attack on Iran but is waiting to give the final order until it becomes clear whether Tehran will agree to terminate its nuclear enrichment program altogether, the outlet wrote, citing three sources familiar with the discussions.

The US president has asked his advisers whether US Massive Ordnance Penetrator (MOP) bombs can penetrate and destroy Iran's Fordow nuclear facility, which is buried deep in a mountain, Axios wrote on Wednesday, citing an anonymous official.

Israel lacks the 30,000-pound (13,600 kg) bunker-busters and has no aircraft capable of carrying them. Nonetheless, Israeli officials have stressed that they will not end the conflict while Fordow remains intact.

Six US B-2 Spirits, which are currently the only bombers capable of carrying the MOP, are deployed within attack range of Iran at the joint UK-US Diego Garcia base, the Financial Times reported on Wednesday. Downing Street is concerned that any strikes by the bombers may draw the UK into the conflict, according to the outlet.

The US has been concentrating more of its forces in the Middle East in recent days, moving in dozens of tanker planes used for refueling jets in flight and air-defense-capable warships, as well as an additional aircraft carrier group.

READ MORE: Iran blames IAEA chief's 'betrayal' for strikes on nuclear sites

Trump told the press on Wednesday that he has "ideas on what to do," but has not made "the final decision." Earlier this week, the president boasted that US jets have unchallenged supremacy in Iranian airspace. He also claimed that killing Supreme Leader Ali Khamenei would be "easy" and demanded Tehran's "unconditional surrender."

Khamenei has replied that Iran "will not capitulate to anyone in the face of coercion."